Adds `crates/wickra-core/tests/invariants.rs` — a property-based (`proptest`) harness asserting three invariants for **every** indicator and bar-builder in the catalogue (every module entry implementing `Indicator` or `BarBuilder`; the lone non-indicator helper `pattern_swing`/`SwingTracker` is excluded by design) — and fixes the non-finite bugs the harness surfaced.
## Invariants
1. **batch == streaming** — `batch()` must replay `update()` exactly.
2. **reset == fresh** — after `reset()`, re-feeding the same data matches a fresh instance.
3. **non-finite rejected without poisoning** (`f64` / `(f64, f64)` families) — a NaN/inf tick returns `None` and leaves state identical to never having seen it.
## How it works
- A single generic `check_seq<I: Indicator>` covers **all** input families — `f64`, `Candle`, `(f64, f64)`, and the exotic `CrossSection`, `Trade`, `DerivativesTick`, `OrderBook`, `TradeQuote` — via per-family `proptest` generators that produce *valid* inputs (e.g. order books are strictly monotonic and uncrossed).
- `check_bars<B: BarBuilder>` covers the bar-builder trait.
- Outputs are compared by `Debug` string so bit-identical `NaN` outputs count as equal — these properties test **determinism**, not NaN-freeness.
## The 38 non-finite fixes
The harness surfaced **38 more** scalar/pairwise indicators that let a NaN/inf tick poison their state — the same class as the 16 pairwise indicators fixed earlier (#251), but missed by the grep-based audit (`kalman_hedge_ratio` and `spread_bollinger_bands` carried `is_finite` on their **constructor** params, not the update input). All 38 now reject non-finite input via the established first-statement guard; signatures unchanged, so every binding inherits the fix. With these in, the non-finite invariant is enforced for every `f64`/`(f64,f64)` indicator going forward — the permanent regression net that would have caught #251.
Warmup-exactness was evaluated and **left out** (not universal — many multi-component/candlestick indicators emit before `warmup_period` by design).
## Verification
- `cargo test -p wickra-core`: 4225 unit + harness + 464 doc/integration, all green.
- `cargo clippy --workspace --all-targets --all-features -- -D warnings`: clean.
- Coverage runs integration tests, so the new guard branches are exercised by the harness's NaN feed.
Also documents the harness in README's Testing section and adds the pending 0.8.4 entries to CHANGELOG `[Unreleased]`.
kingchenc
9973d1a6bf