docs(bench): record remaining gajae optimization deltas

[code-yeongyu]

Summary

• Record WI-5, WI-3, and WI-4 benchmark deltas in bench/IMPROVEMENTS.{md,json}.
• Record WI-2 as deferred because mutation-safety proof did not clear.

Evidence

• WI-5 word-diff: median 22.34375ms -> 8.114834ms, 63.68% improvement, PR perf(coding-agent): fast-path single-line intra-line diffs #41.
• WI-3 compaction-trim: median 29.648125ms -> 12.111666ms, 59.15% improvement, PR perf(coding-agent): prune compaction budget with a running token total #42.
• WI-4 emit-context-clone: median 2.278792ms -> 1.645875ms, 27.77% improvement, PR perf(coding-agent): clone emit context messages via JSON #43.
• WI-2: no code shipped; live agent paths mutate existing AgentMessage objects, so shared-object caching needs deeper proof.

Verification

• node -e 'JSON.parse(require("fs").readFileSync("bench/IMPROVEMENTS.json","utf8")); console.log("json ok")'
• rg -n "Remaining gajae|WI-5|WI-3|WI-4|WI-2" bench/IMPROVEMENTS.md bench/IMPROVEMENTS.json
• git diff --check
• pre-commit npm run check passed.

Plan: plans/port-gajae-3day-remaining-optimizations.md

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Summary by cubic

Documented the remaining Gajae 3‑day optimization results by adding WI‑5 (word‑diff), WI‑3 (compaction‑trim), and WI‑4 (emit‑context‑clone) benchmark deltas with evidence and PR links in bench/IMPROVEMENTS.{md,json}. Marked WI‑2 “mutation safety” as deferred (no code shipped) due to in‑place mutation findings in packages/agent and packages/coding-agent.

^{Written for commit 8719d83. Summary will update on new commits.}