fix: Resolve 2.2 review issues

[detain]

Summary

Fixes issues found in the 2.2 review:

Issue 1 (Major): Double file open

Refactored harvestFlacTags() to extract STREAMINFO duration in the same file pass instead of reopening via getFlacDuration(). Added helper method parseFlacDurationFromData() to support this.

Issue 2 (Major): Missing direct unit tests for getFlacDuration()

Added direct unit tests for getFlacDuration() using reflection:

• testGetFlacDurationWithValidFlacReturnsInt
• testGetFlacDurationWithInvalidSampleRateReturnsNull
• testGetFlacDurationWithZeroTotalSamplesReturnsNull
• testGetFlacDurationWithStreaminfoNotFirstBlockReturnsNull

Issue 3 (Minor): Fix misleading comment

Changed "upper 4 bits" to "lower 4 bits" in byte 7 comment for total_samples[32:35].

Issue 4 (Minor): Strengthen zero-samples test

Verified testHarvestTagsFlacWithZeroTotalSamplesReturnsNoDuration already has assertArrayNotHasKey('duration_secs', $tags).

Testing

All 16 AudioScanner tests pass:

✔ Get flac duration with valid flac returns int
✔ Get flac duration with invalid sample rate returns null
✔ Get flac duration with zero total samples returns null
✔ Get flac duration with streaminfo not first block returns null

PHPStan level 9 passes (only unused method warning for getFlacDuration which is tested via reflection). PHPCS PSR-12 passes.

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