Clarify cache-score read-premium amortization

[ictechgy]

Summary\n- avoid reporting monotonic break-even reuse counts when cache-read multipliers are above uncached cost\n- add max_profitable_reuses for bounded positive read-premium cases\n- add regression coverage for read-premium positive and negative cases\n\n## Validation\n- python3 scripts/sync_plugin_copies.py --check\n- python3 -m py_compile context-guard-kit/cache_score.py plugins/context-guard/bin/context-guard-cache-score tests/test_context_guard_kit.py\n- PYTHONDONTWRITEBYTECODE=1 python3 -m unittest tests.test_context_guard_kit.ClaudeTokenKitTests -k cache_score\n- PYTHONDONTWRITEBYTECODE=1 python3 scripts/prepublish_check.py --skip-tests\n- python3 scripts/release_smoke.py --timeout 20\n- PYTHONDONTWRITEBYTECODE=1 python3 scripts/prepublish_check.py (691 tests)\n- git diff --check\n\nFollow-up to PR #200 final code-review finding.

[ictechgy]

Final ultragoal follow-up evidence for PR #201 (read-premium cache-score amortization)

Reason for follow-up:

• Addresses post-merge PR Harden token savings advisory reports #200 final review finding: cache-read multipliers above uncached cost must not expose a monotonic break-even reuse claim.

Local validation:

• python3 scripts/sync_plugin_copies.py --check ✅
• python3 -m py_compile context-guard-kit/cache_score.py plugins/context-guard/bin/context-guard-cache-score tests/test_context_guard_kit.py ✅
• PYTHONDONTWRITEBYTECODE=1 python3 -m unittest tests.test_context_guard_kit.ClaudeTokenKitTests -k cache_score ✅ (3 tests)
• PYTHONDONTWRITEBYTECODE=1 python3 scripts/prepublish_check.py --skip-tests ✅
• python3 scripts/release_smoke.py --timeout 20 ✅
• PYTHONDONTWRITEBYTECODE=1 python3 scripts/prepublish_check.py ✅ (691 tests)
• git diff --check ✅

Review gates:

• Code review r6: no actionable correctness regressions (.omx/artifacts/token-savings-batch1-pr201-code-review-r6.md) ✅
• Architect review r2: architecturalStatus=CLEAR, blockerCount=0 (.omx/artifacts/token-savings-batch1-pr201-architect-review-r2.md) ✅

CI:

• PR Clarify cache-score read-premium amortization #201 commit 1a24c854028f32c7b8826e3a0f91ca683c6e6d3f GitHub Actions all green: 3.11, 3.12, macOS 3.12 ✅

Release boundary:

• No package release/version mutation in this PR; v0.4.10 remains unchanged. Future source release must be 0.4.11+.