chore(wasm): rayon cfg-gate, event_rx doc, wasm-check CI (deferred from #1007)

[clouatre]

Status update (2026-08-28)

• Item 1 (rayon cfg-gate): re-checked call sites -- par_iter() now appears at 3 sites (analyze.rs:297, cache.rs:97, cache.rs:168), not 5. The underlying blocker (tree-sitter's C build scripts can't cross-compile to wasm32-unknown-unknown) is unchanged and still real.
• Item 2 (event_rx doc comment): stale, drop this item. CodeAnalyzer no longer has an event_rx field, and LogEvent/NeverSessionManager-as-drain-source no longer exist in CodeAnalyzer at all (NeverSessionManager now only appears in main.rs as an unrelated rmcp HTTP session-manager type). Whatever this field was appears to have been refactored away since feat(http): bearer token auth middleware for HTTP MCP transport #1007. Nothing to document here.
• Item 3 (wasm-check CI feature): unaffected by the above; still a real deferred item once tree-sitter WASM support exists. Note the --no-default-features flag in the proposed CI command now only disables the schemars feature (the only remaining feature flag), since the lang-* feature-gate system was removed repo-wide -- languages will still compile in regardless of that flag.

Background

During the implementation of bearer token auth for the HTTP MCP transport (#1007), three
additional changes were drafted and then removed as YAGNI:

1. Rayon cfg-gate in aptu-coder-core (analyze.rs, cache.rs)
2. event_rx doc comment in CodeAnalyzer
3. wasm-check CI feature and job

They are tracked here for when WASM portability becomes a concrete goal.

Items

1. Rayon wasm cfg-gate

analyze.rs and cache.rs use rayon::par_iter() at 5 call sites. Under
#[cfg(target_family = "wasm")] these must fall back to sequential iter(), since rayon
requires OS threads which do not exist in any WASM target.

The implementation is straightforward:

• Gate use rayon::prelude::* with #[cfg(not(target_family = "wasm"))]
• At each par_iter() site, emit sequential iter() under #[cfg(target_family = "wasm")]

Blocker: tree-sitter is an unconditional direct dependency of aptu-coder-core and
includes C build scripts that cannot cross-compile to wasm32-unknown-unknown. The cfg-gate
alone does not make the crate WASM-compilable. Address alongside or after tree-sitter WASM
support.

2. event_rx doc comment

Add a doc comment to the event_rx: Arc<TokioMutex<Option<mpsc::UnboundedReceiver<LogEvent>>>>
field in CodeAnalyzer explaining the drain-once invariant under NeverSessionManager.

Small, self-contained. Can ship independently of item 1.

3. wasm-check CI feature and job

Because cargo check --target wasm32-unknown-unknown is not viable (tree-sitter C build
scripts, Rust 1.96 explicit_builtin_cfgs_in_flags blocks RUSTFLAGS='--cfg target_family="wasm"'),
a wasm-check Cargo feature was designed to activate the wasm cfg branches on a native host:

# aptu-coder-core/Cargo.toml
wasm-check = []

# ci.yml
- run: cargo check --locked -p aptu-coder-core --no-default-features --features wasm-check

This requires updating all cfg gates to any(target_family = "wasm", feature = "wasm-check"),
adding permanent indirection at every site. Ship together with item 1.

Prerequisites

• tree-sitter WASM support in the Rust ecosystem (upstream, no timeline)
• Concrete WASM deployment target chosen (wasm32-wasip2 + wasmtime/Spin recommended over
  wasm32-unknown-unknown + Cloudflare Workers; see prior analysis)