tests: missed piecereader test

[magik6k]

Used this to repro the recent piecereader issues but apparently didn't commit with that PR.

Still should be useful as it covers the existing logic pretty comprehensively so any future changes/optimizations will also benefit.

---

Note

Introduces a full test suite for pieceReader to validate correctness and performance across common and edge scenarios.

• Covers Read, Seek, and ReadAt semantics, including position tracking and EOF/partial reads
• Verifies LRU caching behavior for small reads, header caching at offset 0, and read-ahead
• Tests "burn bytes" optimization vs. new reader creation on seeks (forward/backward)
• Ensures thread safety with concurrent Read/Seek and ReadAt operations
• Exercises error handling from getReader, reader reinitialization, and close/double-close cases
• Adds benchmarks for sequential and random access patterns

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[LexLuthr]

@magik6k Need to fix lint with defer in a func. Rest is good.

[LexLuthr]

@magik6k Can you please fix the lint so we can merge this?