feat: add Market Breadth family with CrossSection input (#153)

## What

Adds a new indicator input type and family for **market-breadth** analysis — indicators that aggregate the state of an entire universe of symbols at each tick, rather than a single instrument's price. This is the last open input-type on the expansion roadmap (S10) and unblocks the remaining breadth indicators (McClellan, TRIN, High-Low Index, ...).

## Core

- **`CrossSection` input type** (`crates/wickra-core/src/cross_section.rs`) — one tick carrying the per-symbol state of the whole universe as a `Vec<Member>` + `timestamp`. Each `Member` precomputes a signed `change` (sign classifies advancing / declining / unchanged), a `volume`, and `new_high` / `new_low` extreme flags, so the breadth indicators stay stateless per tick. Both `Member` and `CrossSection` are `#[non_exhaustive]` for additive field growth. `CrossSection::new` validates the universe (non-empty, finite changes, finite non-negative volumes); `new_unchecked` skips validation for hot paths. `advancers()` / `decliners()` count by sign.
- **`Error::InvalidCrossSection`** variant for the validation failures.
- **`AdvanceDecline`** (`advance_decline.rs`) — the Advance/Decline Line: the running cumulative sum of net advancing-minus-declining issues. `Input = CrossSection`, `Output = f64`, ready after the first tick.
- New **"Market Breadth"** `FAMILIES` group; indicator count **314 → 315**, family count nineteen → twenty.

## Bindings

All custom (CrossSection is non-scalar, so no macros apply). The universe crosses each boundary as parallel arrays (`change`, `volume`, `new_high`, `new_low`):
- **Python / Node** expose `update` + `batch` (one array group per tick). Node satisfies the completeness contract (`update`/`batch`/`reset`/`isReady`/`warmupPeriod`).
- **WASM** exposes only `update` (the universe is ragged across ticks, matching the other multi-input wasm indicators) with numeric high/low flags.
- Python `map_err` gains the new error arm; `__init__.py` gets a `# Market Breadth` section in both the import and `__all__` blocks. `index.d.ts` / `index.js` regenerated.

## Tests / Fuzz

- Dedicated **streaming-vs-batch + reference-value + ragged-rejection** tests in Python (`test_new_indicators.py`) and Node (`indicators.test.js`) — kept out of the scalar/candle parametrize lists.
- Rust unit tests cover every reject branch (empty / non-finite change / negative & non-finite volume) and every indicator branch.
- New fuzz target `indicator_update_crosssection` drives `AdvanceDecline` over bounded ragged universes built with `new_unchecked`.

## Verify

- `cargo fmt --all` clean
- `cargo test -p wickra-core --lib` → 2593 passed; `--doc` → 298 passed
- `cargo clippy --workspace --all-targets --all-features -- -D warnings` clean
- `cd bindings/node && npm run build && npm test` → 398 passed
- `maturin develop --release` + `pytest bindings/python/tests` → all passed
- counter check: mod-count 315 == lib-block 315

crates/wickra-core/src/indicators/advance_decline.rs

@@ -0,0 +1,168 @@
+//! Advance/Decline Line — cumulative net advancing-minus-declining issues.
+
+use crate::cross_section::CrossSection;
+use crate::traits::Indicator;
+
+/// Advance/Decline Line (A/D Line) — the running cumulative sum of net advancing
+/// issues across a universe.
+///
+/// On each [`CrossSection`] tick the net breadth is `advancers - decliners`:
+/// the number of symbols with a positive price change minus the number with a
+/// negative change (unchanged symbols are ignored). The line accumulates this
+/// net value over time, so a rising line means advancers have persistently
+/// outnumbered decliners — broad participation — while a falling line warns that
+/// a rally is being carried by fewer and fewer names (a breadth divergence when
+/// the index itself is still rising).
+///
+/// `Input = CrossSection`, `Output = f64`. The line is defined from the very
+/// first tick, so `warmup_period == 1` and the indicator is ready after one
+/// update.
+///
+/// # Example
+///
+/// ```
+/// use wickra_core::{AdvanceDecline, CrossSection, Indicator, Member};
+///
+/// let mut ad = AdvanceDecline::new();
+/// // 3 advancers, 1 decliner -> net +2.
+/// let tick = CrossSection::new(
+///     vec![
+///         Member::new(1.0, 10.0, false, false),
+///         Member::new(0.5, 10.0, false, false),
+///         Member::new(2.0, 10.0, false, false),
+///         Member::new(-1.0, 10.0, false, false),
+///     ],
+///     0,
+/// )
+/// .unwrap();
+/// assert_eq!(ad.update(tick), Some(2.0));
+/// ```
+#[derive(Debug, Clone, Default)]
+pub struct AdvanceDecline {
+    line: f64,
+    has_emitted: bool,
+}
+
+impl AdvanceDecline {
+    /// Construct a new Advance/Decline Line indicator.
+    #[must_use]
+    pub const fn new() -> Self {
+        Self {
+            line: 0.0,
+            has_emitted: false,
+        }
+    }
+}
+
+impl Indicator for AdvanceDecline {
+    type Input = CrossSection;
+    type Output = f64;
+
+    fn update(&mut self, section: CrossSection) -> Option<f64> {
+        let net = section.advancers() as f64 - section.decliners() as f64;
+        self.line += net;
+        self.has_emitted = true;
+        Some(self.line)
+    }
+
+    fn reset(&mut self) {
+        self.line = 0.0;
+        self.has_emitted = false;
+    }
+
+    fn warmup_period(&self) -> usize {
+        1
+    }
+
+    fn is_ready(&self) -> bool {
+        self.has_emitted
+    }
+
+    fn name(&self) -> &'static str {
+        "AdvanceDecline"
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::cross_section::Member;
+    use crate::traits::BatchExt;
+
+    /// Build a cross-section with `up` advancers, `down` decliners and `flat`
+    /// unchanged symbols.
+    fn section(up: usize, down: usize, flat: usize) -> CrossSection {
+        let mut members = Vec::new();
+        for _ in 0..up {
+            members.push(Member::new(1.0, 10.0, false, false));
+        }
+        for _ in 0..down {
+            members.push(Member::new(-1.0, 10.0, false, false));
+        }
+        for _ in 0..flat {
+            members.push(Member::new(0.0, 10.0, false, false));
+        }
+        CrossSection::new(members, 0).unwrap()
+    }
+
+    #[test]
+    fn accessors_and_metadata() {
+        let ad = AdvanceDecline::new();
+        assert_eq!(ad.name(), "AdvanceDecline");
+        assert_eq!(ad.warmup_period(), 1);
+        assert!(!ad.is_ready());
+    }
+
+    #[test]
+    fn first_tick_emits_net_breadth() {
+        let mut ad = AdvanceDecline::new();
+        assert_eq!(ad.update(section(3, 1, 0)), Some(2.0));
+        assert!(ad.is_ready());
+    }
+
+    #[test]
+    fn line_accumulates_across_ticks() {
+        let mut ad = AdvanceDecline::new();
+        assert_eq!(ad.update(section(3, 1, 0)), Some(2.0)); // +2 -> 2
+        assert_eq!(ad.update(section(1, 4, 0)), Some(-1.0)); // -3 -> -1
+        assert_eq!(ad.update(section(2, 0, 0)), Some(1.0)); // +2 -> 1
+    }
+
+    #[test]
+    fn unchanged_symbols_are_ignored() {
+        let mut ad = AdvanceDecline::new();
+        // 2 up, 2 down, 5 unchanged -> net 0, line stays flat.
+        assert_eq!(ad.update(section(2, 2, 5)), Some(0.0));
+        assert_eq!(ad.update(section(2, 2, 5)), Some(0.0));
+    }
+
+    #[test]
+    fn reset_clears_state() {
+        let mut ad = AdvanceDecline::new();
+        ad.update(section(5, 0, 0));
+        assert!(ad.is_ready());
+        ad.reset();
+        assert!(!ad.is_ready());
+        // Line restarts from zero, not from the pre-reset value.
+        assert_eq!(ad.update(section(1, 0, 0)), Some(1.0));
+    }
+
+    #[test]
+    fn batch_equals_streaming() {
+        let sections = vec![
+            section(3, 1, 2),
+            section(1, 4, 0),
+            section(2, 2, 1),
+            section(5, 0, 3),
+        ];
+        let mut a = AdvanceDecline::new();
+        let mut b = AdvanceDecline::new();
+        assert_eq!(
+            a.batch(&sections),
+            sections
+                .iter()
+                .map(|s| b.update(s.clone()))
+                .collect::<Vec<_>>()
+        );
+    }
+}

crates/wickra-core/src/indicators/mod.rs

@@ -11,6 +11,7 @@ mod ad_oscillator;
 mod adaptive_cycle;
 mod adl;
 mod advance_block;
+mod advance_decline;
 mod adx;
 mod adxr;
 mod alligator;
@@ -325,6 +326,7 @@ pub use ad_oscillator::AdOscillator;
 pub use adaptive_cycle::AdaptiveCycle;
 pub use adl::Adl;
 pub use advance_block::AdvanceBlock;
+pub use advance_decline::AdvanceDecline;
 pub use adx::{Adx, AdxOutput};
 pub use adxr::Adxr;
 pub use alligator::{Alligator, AlligatorOutput};
@@ -1038,6 +1040,7 @@ pub const FAMILIES: &[(&str, &[&str])] = &[
         "Alt-Chart Bars",
         &["RenkoBars", "KagiBars", "PointAndFigureBars"],
     ),
+    ("Market Breadth", &["AdvanceDecline"]),
 ];
 
 #[cfg(test)]
@@ -1066,6 +1069,6 @@ mod family_tests {
         // the actual indicator count is the early-warning signal that an
         // indicator was added without being assigned a family.
         let total: usize = FAMILIES.iter().map(|(_, ns)| ns.len()).sum();
-        assert_eq!(total, 314, "FAMILIES total drifted from indicator count");
+        assert_eq!(total, 315, "FAMILIES total drifted from indicator count");
     }
 }