feat(momentum): bool-param demo strategy + aura sweep --strategy selector

Proves the #105 generic portable member key end-to-end: a demo strategy whose
params have nothing in common with the SMA-cross demo, swept by the same
machinery, with a bool axis surfacing — conformant — in the member dir names.

- momentum strategy: price -> Ema(length:i64) -> Sub(price-ema) ->
  Exposure(scale:f64) -> LongOnly(enabled:bool) -> SimBroker. Three swept params
  of three kinds incl. a bool; the exposure sink taps the post-gate exposure so
  the bool's effect is in the trace.
- aura sweep gains --strategy <sma|momentum> via a pure, unit-tested
  parse_sweep_args (mirroring parse_real_args); default = SMA-cross, so the
  existing bare / --name / --trace forms are unchanged.
- `aura sweep --strategy momentum --trace mom` persists 8 member dirs named e.g.
  ema.length-5_exposure.scale-0.5_longonly.enabled-true — the bool in the dir
  name, every name matching [A-Za-z0-9._-], each chartable.
- ledger (docs/design/INDEX.md): the C22/#101 member-key note's sweep clause
  amended to the generalised portable form (MC seed{N} / walk-forward oos{ns}
  unchanged).

Self-verified: cargo build --workspace, cargo test --workspace (all green incl.
momentum_param_space_is_ema_exposure_longonly, the 8-point determinism test,
the parse_sweep_args grammar test, and the momentum_sweep_trace integration test
asserting 8 portable bool-bearing dirs + a chartable member), cargo clippy
--workspace --all-targets -D warnings.

refs #105

docs/design/INDEX.md

@@ -1030,7 +1030,10 @@ to family runs: `aura sweep|mc|walkforward --trace <name>` persists *each member
 a nested standalone run-dir `runs/traces/<name>/<member_key>/`, reusing
 `persist_traces`/`TraceStore` verbatim (engine untouched — persistence is a
 side-effect inside each per-member closure). The `member_key` is content-derived and
-deterministic — sweep `f{fast}s{slow}`, MC `seed{N}`, walk-forward `oos{ns}` — never
+deterministic — sweep: the *varying* axes rendered as a filesystem-portable
+directory component (`<axis-name>-<value>` tokens joined by `_`, charset
+`[A-Za-z0-9._-]`, case-less values, length-capped with an FNV fallback), MC
+`seed{N}`, walk-forward `oos{ns}` — never
 a runtime ordinal, because members run in parallel under the engine's `Fn + Sync`
 HOFs, where a counter would be schedule-dependent (C1); concurrent `TraceStore::write`
 targets disjoint member dirs, so it is lock-free. Opt-in: without `--trace`,