perf: avoid buffering stream when computing placeholder PNG C2PA chunk

[ok-nick]

Reduces memory usage when reading/signing a PNG that doesn't contain a C2PA manifest by optimizing get_object_locations_from_stream to quickly find where the C2PA chunk goes rather than computing it via write_cai. When signing a 100MB PNG, the total memory usage decreases by 29.4%.

Before:

After:

[codspeed-hq]

Merging this PR will improve performance by 11.96%

⚡ 1 improved benchmark
✅ 29 untouched benchmarks
⏩ 64 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	small	25.2 ms	22.5 ms	+11.96%

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing ok-nick/png-mem-opt (223f661) with main (a48f1cc)}

Footnotes

1. 64 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[codspeed-hq]

Merging this PR will improve performance by 12.06%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 1 improved benchmark
✅ 29 untouched benchmarks
⏩ 64 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	small	25.2 ms	22.5 ms	+12.06%

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing ok-nick/png-mem-opt (48f1ae2) with main (350e138)}

Footnotes

1. 64 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[gpeacock]

I'm wondering if there are side effects caused by modifying the input stream. We probably don't expect that. It is mut so that we can seek, not modify.

[ok-nick]

@gpeacock I'm not sure what you mean. The input stream is read-only and this change only seeks?

[ok-nick]

Discussed offline with Gavin.