F13c: restructure the indicator catalogue into eight families

The original taxonomy was four classical families plus a statistics group,
with the F1-F12 expansion slotted in as sub-categories. This regroups the
whole 71-indicator catalogue into eight top-level families, each with at
least five members:

  Moving Averages (12), Momentum Oscillators (13), Trend & Directional (9),
  Price Oscillators (5), Volatility & Bands (12), Trailing Stops (5),
  Volume (9), Price Statistics (7).

- Wiki: docs/wiki/indicators/ reorganised into eight family folders; all 71
  indicator pages moved with `git mv`. Every internal cross-link is
  normalised to `../<family>/Indicator-X.md`, each page's `Family` field is
  set to its new family, and two pre-existing `../Indicator-Chaining.md`
  links (should have been `../../`) are corrected. A link check confirms
  every relative wiki link resolves.
- Indicators-Overview.md fully rewritten around the eight families;
  Home.md indicator reference and the README family table follow suit.
- Warmup-Periods.md gains the eight F13 indicators; CHANGELOG records the
  46-indicator expansion (25 -> 71) and the eight-family taxonomy.
- Tests: Node indicators.test.js and Python test_new_indicators.py cover
  all eight new indicators (Node 91/91, Python 117/117 green).

cargo fmt + clippy (core/wickra/data/wasm/node) clean; 508 core tests,
25 data tests and 74 doctests green.

docs/wiki/indicators/price-oscillators/Indicator-Ppo.md

@@ -0,0 +1,154 @@
+# PPO
+
+> Percentage Price Oscillator — MACD expressed as a percentage of the slow
+> EMA, so readings are comparable across instruments.
+
+## Quick reference
+
+| Field | Value |
+|-------|-------|
+| Family | Price Oscillators |
+| Input type | `f64` (single close) |
+| Output type | `f64` |
+| Output range | unbounded around zero (percent) |
+| Default parameters | `(fast = 12, slow = 26)` (Python) |
+| Warmup period | `slow` |
+| Interpretation | Percentage gap between a fast and slow EMA; zero-line crosses are signals. |
+
+## Formula
+
+```
+PPO = 100 · (EMA_fast − EMA_slow) / EMA_slow
+```
+
+PPO is [`MacdIndicator`](../trend-directional/Indicator-MacdIndicator.md) divided by the slow
+EMA. That single change makes it **scale-free**: a `PPO` of `1.5` always
+means "the fast EMA is 1.5 % above the slow EMA", whether the instrument
+trades at $5 or $5000 — so PPO values can be compared across assets and
+across time, which raw MACD values cannot. The classic PPO **signal
+line** is a 9-period EMA of this PPO line; compose it with
+[`Chain`](../../Indicator-Chaining.md) and an `Ema(9)`.
+
+## Parameters
+
+| Name   | Type    | Default       | Valid range      | Description |
+|--------|---------|---------------|------------------|-------------|
+| `fast` | `usize` | `12` (Python) | `>= 1`, `< slow` | Fast EMA period. |
+| `slow` | `usize` | `26` (Python) | `> fast`         | Slow EMA period. |
+
+`fast` must be strictly less than `slow` — otherwise `new` returns
+`Error::InvalidPeriod`. A zero period returns `Error::PeriodZero`. The
+Python binding defaults the pair to `(12, 26)`; the `periods` property
+returns `(fast, slow)`.
+
+## Inputs / Outputs
+
+From `crates/wickra-core/src/indicators/ppo.rs`:
+
+```rust
+impl Indicator for Ppo {
+    type Input = f64;
+    type Output = f64;
+    // update(&mut self, input: f64) -> Option<f64>
+}
+```
+
+A single `f64` close in, an `Option<f64>` out. Python maps this to
+`float | None` / `numpy.ndarray` (NaN warmup); Node to `number | null` /
+`Array<number>` (NaN warmup).
+
+## Warmup
+
+`Ppo::new(fast, slow).warmup_period() == slow`. Both EMAs are SMA-seeded;
+the slow EMA is the last to seed, at input `slow`, which is also when PPO
+emits its first value.
+
+## Edge cases
+
+- **Constant series.** Both EMAs converge to the constant, so their gap —
+  and PPO — is `0` (`constant_series_yields_zero` pins this).
+- **Zero slow EMA.** A `0.0` slow EMA would divide by zero; PPO reports
+  `0.0` for that bar instead.
+- **NaN / infinity inputs.** Non-finite inputs are silently dropped; the
+  EMAs are not advanced.
+- **Reset.** `ppo.reset()` clears both EMAs and the cached value.
+
+## Examples
+
+### Rust
+
+```rust
+use wickra::{BatchExt, Indicator, Ppo};
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let mut ppo = Ppo::new(12, 26)?;
+    let prices: Vec<f64> = (1..=80).map(f64::from).collect();
+    let out = ppo.batch(&prices);
+    println!("warmup_period = {}", ppo.warmup_period());
+    println!("last > 0: {}", out.last().unwrap().unwrap() > 0.0);
+    Ok(())
+}
+```
+
+Output:
+
+```
+warmup_period = 26
+last > 0: true
+```
+
+In a rising series the fast EMA leads the slow EMA, so PPO is positive.
+
+### Python
+
+```python
+import numpy as np
+import wickra as ta
+
+ppo = ta.PPO()  # (fast=12, slow=26)
+prices = np.full(60, 100.0)  # flat series
+print(ppo.batch(prices)[-1])  # both EMAs equal -> 0
+```
+
+Output:
+
+```
+0.0
+```
+
+### Node
+
+```javascript
+const ta = require('wickra');
+const ppo = new ta.PPO(12, 26);
+const prices = Array.from({ length: 80 }, (_, i) => 100 + i);
+console.log('warmupPeriod:', ppo.warmupPeriod());
+```
+
+## Interpretation
+
+`Ppo` is read exactly like MACD: the zero-line cross (fast EMA crossing
+the slow EMA), the signal-line cross (PPO crossing its own 9-EMA), and
+histogram-style divergence. Its advantage over MACD is comparability — a
+PPO scan across a watchlist ranks instruments by *relative* trend
+strength, which a MACD scan cannot do because MACD is in each
+instrument's own price units.
+
+## Common pitfalls
+
+- **Expecting a bundled signal line.** `Ppo` here is the single PPO line;
+  add `Ema(9)` via `Chain` for the signal line and histogram.
+- **`fast >= slow`.** The constructor rejects it — the fast EMA must be
+  the faster one.
+
+## References
+
+Gerald Appel's MACD, re-expressed as a percentage. The implementation
+follows the standard PPO definition and matches TA-Lib's `PPO`.
+
+## See also
+
+- [Indicator-MacdIndicator.md](../trend-directional/Indicator-MacdIndicator.md) — the price-unit
+  original, with a bundled signal line and histogram.
+- [Indicator-Ema.md](../moving-averages/Indicator-Ema.md) — the underlying average.
+- [Indicators-Overview.md](../../Indicators-Overview.md) — the full taxonomy.