chore(bench): add --pad-entries to cache_hit_rate for threshold sweeps

[engkimo]

Summary

• Add --pad-entries N flag to benchmarks/cache_hit_rate.py so the --verify-wiring pad size can be tuned independently of the default 160 entries (~8.4K tokens, ~50 tok/entry).
• Used to binary-search Claude Haiku 4.5's empirical prompt-cache minimum.

Finding (TD-193 follow-up — Haiku 4.5 threshold pinned)

pad_entries	prompt_tok	result
60	3475	MISS
70	3965	MISS
72	4063	MISS
73	4112	MISS
74	4161	HIT
80	4455	HIT
100	5435	HIT
160	8375	HIT

Empirical Claude 4 cache minimums (this account):

• Sonnet 4.6 ≈ 2048 tokens (2^11)
• Haiku 4.5 ≈ 4096 tokens (2^12)

Both clean powers of 2; documented 1024-token minimum is wrong for both.

Test plan

• ruff check benchmarks/cache_hit_rate.py — clean.
• Live: 8 sweeps against real Anthropic API (claude-haiku-4-5-20251001); each --calls 3 --verify-wiring --pad-entries N. Results above.

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 0a36c90b-23e6-49d3-8624-09b151f4e3ab

📥 Commits

Reviewing files that changed from the base of the PR and between 97f11a7 and f41a621.

📒 Files selected for processing (1)

• benchmarks/cache_hit_rate.py

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch chore/cache-bench-pad-entries

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