1.13.0 — preserve unknown cache usage from older ledgers - #41

Summary

• version new usage-ledger rows so measured cache zeros are distinguishable from historical synthetic zeros
• conservatively report cache usage from v1 turn rows as unknown
• cover v1 explicit zero and v2 measured zero in unit and CLI fixtures

Test plan

• cargo fmt -- --check
• env -u OPENAI_API_KEY -u OPENAI_BASE_URL cargo test --locked --all-targets --all-features (703 unit tests, 95 CLI tests)
• env -u OPENAI_API_KEY -u OPENAI_BASE_URL cargo clippy --locked --all-targets --all-features -- -D warnings
• git diff --check

No paid live Codex/cache test was run.

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