perf(runtime): make context pruning size-aware

[yashranaway]

Closes #25

Summary

• measure each result and candidate once instead of repeatedly serializing the full response
• prune the largest entries regardless of position while retaining stable order
• byte-budget text fallback with a bounded prefix search
• report exact finalized byte/token statistics and preserve fail-closed bounds
• add jsdom regressions for mid-array pruning, serialization count, and text fallback
• mark backlog B5 complete

Validation

• npx --yes pnpm@9.15.0 test:runtime
• npx --yes pnpm@9.15.0 test:e2e:linux (includes 43/43 protocol tests)