perf: O(H) lca() depth-walk + dispatch bench scoreboard

The runtime `lca()` helper used a `heapless::Vec<u16,32>` ancestry-set
with a linear `contains()` per node — O(H²) and capped at H ≤ 32. The
codegened `__DEPTH` table already had everything needed for an O(H)
depth-walk with no allocation and no hierarchy ceiling. Switch to that.

Also adds an iai-callgrind dispatch bench harness (Linux-only dev-dep)
so future optimizations can be measured deterministically by instruction
count. Seven benches cover: cold start, lateral H=2/4/8, up, self, and a
self-emit chain. Baseline numbers checked in at `hsmc/benches/baseline.txt`.

The latency analysis (`docs/003. latency-analysis.md`) is rounded out
with event-entry-point and cooperative-driving audits — three new
sections documenting the four event injection paths, the run-loop's
cooperative shape, and three correctness invariants (sticky overflow
flag, biased-select trap, cancel-safety on durings).

Bench deltas (cargo bench --bench dispatch -- --baseline=pre_lca_fix):

  h2_cold_start        -1.9%  /  -0.3%   (priming step, no lca() — noise)
  h2_warm_lateral     -22.8%  / -22.1%
  h4_cross_tree       -25.9%  / -10.6%
  h4_up                +1.0%  /  +0.4%   (up-transition, no lca() — noise)
  h4_self             -21.5%  / -11.7%
  h8_cross_tree       -46.2%  / -41.0%   ← headline: deepest LCA stress
  chain_self_emit     -15.4%  / -10.4%

Improvement scales with hierarchy depth, exactly as the O(H²) → O(H)
algorithmic change would predict.

All tests pass under --no-default-features, --features tokio, and
--features embassy.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Cargo.lock

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