perf: preallocate sourcemap mappings encoder

[SyMind]

No description provided.

[codspeed-hq]

Merging this PR will degrade performance by 13.99%

❌ 3 regressed benchmarks
✅ 10 untouched benchmarks
⏩ 7 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
❌	repetitive_react_components_map	1.7 ms	1.9 ms	-10.63%
❌	complex_replace_source_map	8.2 ms	11 ms	-25.92%
❌	concat_generate_string	43.8 µs	45.6 µs	-3.91%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing estimated_mappings_capacity (00c9244) with main (f86ace1)}

Footnotes

1. 7 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. ↩