src: avoid copying SEA snapshot data

[colinhacks]

Description

When a single executable application uses a startup snapshot, Node.js currently copies the complete V8 startup-data region out of the embedded SEA resource during deserialization. The resource remains mapped for the process lifetime, so that copy is unnecessary.

This change lets the SEA loader borrow the V8 startup data directly from the executable image. File-backed and embedder-provided snapshot blobs keep the existing owned-copy behavior because their input buffers do not have the required lifetime.

Results

The following results are medians from 240 randomized, interleaved launches after 30 warmup launches on Linux x64 (Intel Xeon Platinum 8481C). The baseline and patched executables were built from 6f41e415639b5ec3dd816e44945cc73b4d7651e3.

SEA workload	Before	After	Difference
Empty snapshot	28.335 ms	26.610 ms	-1.725 ms (-6.1%)
Computation in snapshot	28.131 ms	26.491 ms	-1.640 ms (-5.8%)
100,000 objects in snapshot	56.285 ms	51.541 ms	-4.745 ms (-8.4%)

Validation

• make lint-cpp
• python3 tools/test.py --mode=release sea
• make -j22 test-only

AI disclosure: a coding agent prepared the investigation, patch, measurements, and description under the contributor's direction. The agent verified the lifetime invariant against the source, built Node.js, and ran the validation above. The contributor approved opening this pull request and will respond personally during review.

[nodejs-github-bot]

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• @nodejs/startup

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[MikeMcC399]

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