fix(compaction): unexport metric fields to prevent mutex bypass

[xe-nvdk]

Summary

Unexports metric fields on Manager and BaseTier that were previously exported, allowing direct reads/writes that bypass the mutex-protected Stats() / GetBaseStats() accessors.

Closes #350.

Changes

• internal/compaction/manager.go: TotalJobsCompleted → totalJobsCompleted, TotalJobsFailed → totalJobsFailed, TotalFilesCompacted → totalFilesCompacted, TotalBytesSaved → totalBytesSaved, TotalManifestsRecov → totalManifestsRecov
• internal/compaction/tier.go: TotalCompactions → totalCompactions, TotalFilesCompacted → totalFilesCompacted, TotalBytesSaved → totalBytesSaved
• RELEASE_NOTES_2026.06.2.md: Added bug fix entry

All usages are internal to the compaction package.

Test Plan

• go build ./internal/compaction/... — clean
• go build ./cmd/... ./internal/... — clean (no external callers)
• go test ./internal/compaction/... -v — all pass
• Deep review with configuration matrix — no findings

[gemini-code-assist]

Code Review

This pull request unexports the compaction metric fields in both the Manager and BaseTier structs to prevent data races from external access, routing all reads through their respective mutex-protected stats methods. The reviewer recommended introducing a thread-safe helper method on BaseTier to manage updates to these metrics, ensuring that package-level writes do not bypass the mutex and cause data races.

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[xe-nvdk]

Addressed the finding — added RecordCompaction(filesCompacted int, bytesSaved int64) thread-safe helper on BaseTier so tier implementations within the package cannot bypass t.mu when incrementing metrics.

@github-copilot-code-review[bot] re-review please.

[xe-nvdk]

@gemini-code-assist re-review please — added RecordCompaction helper on BaseTier per your finding.

[gemini-code-assist]

Code Review

This pull request unexports compaction metric fields in both the Manager and BaseTier structs to prevent potential data races from unprotected access. It also introduces a thread-safe RecordCompaction helper method on BaseTier to safely increment metrics under a mutex lock, and updates the release notes accordingly. As there are no review comments, I have no feedback to provide.

Important

The consumer version of Gemini Code Assist on GitHub is being sunset. Starting June 18, 2026, new organization installations will be blocked, and all code review activity will officially cease on July 17, 2026.
For more details on the timeline and next steps, please review the Help Documentation.