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/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");
     }
 }