Retier post-change validation to per-change/per-milestone cadence

[quiet-node]

Summary

Post-Change Validation now runs on two tiers instead of one. Every code change gets a fast, scoped check (targeted cargo test --lib <module>::, scoped cargo clippy/cargo fmt --check, and for frontend changes eslint on the touched files plus tsc --noEmit, or the new validate-build:fast script as a single convenience command). The full bun run test:all:coverage + bun run validate-build gates run once per feature, PR, or session end, not after every intermediate edit.

This only changes cadence, not the bar: a feature or PR still isn't done until the full gates are green, and CI keeps enforcing the 100% coverage requirement on every PR exactly as before. The Testing section's coverage mandate is untouched; only the "always run test:all:coverage" line was scoped to milestone/PR time instead of every edit.

Key changes

• Rewrote ## Post-Change Validation in CLAUDE.md into explicit per-change (fast) and per-milestone (full) tiers.
• Scoped the Testing section's test:all:coverage reminder to milestone/PR time.
• Added validate-build:fast to package.json (lint + format:check + typecheck + debug cargo build, no bundle/sign) as the convenience command for the per-change tier.

Testing

• bun run format:check passes (docs/config-only change, no source files touched).