fix: cache breakable fit advances per mode

[hiSandog]

Summary

• cache breakable fit advances separately for each fit mode
• preserve the long segment-prefix fallback under the pair-context cache slot
• add a regression test covering repeated calls across sum, pair, and prefix modes

Fixes #137.

Validation

• npx tsc --noEmit
• npx oxlint --type-aware src
• git diff --check

Note: bun is not installed in this environment, so I could not run bun test src/layout.test.ts.