Pin the WASM toolchain and re-tighten the CI size guard

[dean0x]

Location: .github/workflows/ci.yml:97-108

The WASM size budget is on its 4th raise (700K→750K→800K), each in the PR that caused the growth, so it documents bloat rather than catching it.

Deferred From: PR #171 (Closes #61) — mds lint release gate
Review: .devflow/docs/reviews/feat-mds-lint-61/2026-07-11_1046/resolution-summary.md

Reason for Deferral:
This is a follow-up effort, not a change to the current PR. Must be done after release stabilization.

Acceptance Criteria:

• Pin wasm-pack version (e.g., 1.3.x) in CI setup
• Pin Binaryen version (e.g., v129) in .github/actions/setup-wasm/
• Measure current WASM size on main (post v0.4.0)
• Re-tighten guard to measured size + 5-10KB headroom
• Document rationale in ci.yml comments