read: TS/JS structural summary stays raw under the heuristic folder; evaluate tree-sitter WASM grammars (owner decision)

## Summary
Follow-up to #1639 (read structural default summary). The shipped folder is a brace-scanner heuristic gated by a signature-safety oracle and a bake-off threshold (candidate median token saving must reach 90% of the omp ReadTool reference per language). After the safety corrections in #1644 the heuristic qualifies only for JSON; TypeScript (40.3% vs 47.3% threshold) and JavaScript fall below the bar and are frozen `raw` with `rawReasons: wasm_candidate_pending_owner`.

## Proposal
Evaluate a tree-sitter WASM grammar path for TS/TSX/JS (and optionally Python/Rust/Go) as the fold-boundary source, keeping the same oracle, threshold rule, and frozen-selection mechanism:
- measure bundle size delta and cold-start cost per grammar against the release binary budget;
- run the existing bake-off with the WASM candidate as a second engine and freeze per-language selections from the numbers;
- keep the heuristic as fallback where the grammar is absent or fails to parse.

## Owner decision required
Whether to accept the binary-size increase for WASM grammars (decision gate OQ1 in the original plan defaulted to "no WASM" when unanswered). Nothing in this issue changes the default until the owner decides.

## Related
- #1639 / #1644 (S1 delivery; frozen selection receipt tracked under packages/agent/test/harness/fixtures/read-summary/)