src: avoid copying SEA snapshot data

[colinhacks]

Description

SEA snapshots live in a resource mapped from the executable. Node.js currently copies the V8 startup data out of that resource when it loads the snapshot.

This change lets V8 read the data in place. Snapshots loaded from a file or through the embedder API still use a copy because those buffers may not live long enough.

Benchmark

Median of 240 launches in randomized order, after 30 warmups. Linux x64 on an Intel Xeon Platinum 8481C. Both executables were built from 6f41e415639b5ec3dd816e44945cc73b4d7651e3.

SEA snapshot	Before	After	Change
Empty	28.335 ms	26.610 ms	-1.725 ms (-6.1%)
Computation	28.131 ms	26.491 ms	-1.640 ms (-5.8%)
100,000 objects	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: I used a coding agent for the investigation, patch, benchmarks, and PR text. It checked the resource lifetime against the source, built Node.js, and ran the tests above. I approved the PR and will handle review comments myself.

[nodejs-github-bot]

Review requested:

• @nodejs/startup

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[codecov]

Codecov Report

❌ Patch coverage is 86.95652% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.19%. Comparing base (718cfe4) to head (ab51407).
⚠️ Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
src/node_snapshotable.cc	90.00%	1 Missing and 1 partial ⚠️
src/node.cc	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65876      +/-   ##
==========================================
- Coverage   92.37%   90.19%   -2.18%     
==========================================
  Files         417      771     +354     
  Lines      188656   264921   +76265     
  Branches    28838    50336   +21498     
==========================================
+ Hits       174268   238950   +64682     
- Misses      14051    16954    +2903     
- Partials      337     9017    +8680     

Files with missing lines	Coverage Δ
src/env.h	98.38% <100.00%> (ø)
src/node.cc	76.65% <50.00%> (ø)
src/node_snapshotable.cc	73.59% <90.00%> (ø)

... and 482 files with indirect coverage changes

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