src: allow building the snapshot on top of a V8 startup blob

[codebytere]

CommonEnvironmentSetup::CreateForSnapshotting() always creates the SnapshotCreator without a startup blob, so V8 sets its heap up from scratch. An embedder whose V8 is built with external startup data (Chromium-style: only the deserializer is linked in) can't do that at all, so node_mksnapshot and the embedder snapshot API are unusable there without patching Node; and even where it works, a from-scratch snapshot sits on a different read-only-heap lineage than the isolates the embedder creates from its own blob, which V8's shared RO heap doesn't allow in one process.

This adds SnapshotConfig::base_blob, an optional caller-owned v8::StartupData that is passed to the SnapshotCreator (CreateParams::snapshot_blob), so V8 deserializes its heap from that blob and Node adds its isolate data and contexts on top - the same way Blink's context snapshot is produced. node_mksnapshot grows --v8-snapshot-blob=<file> for hosts that build Node against such a V8. Node's own build never sets it and is unchanged; consuming a snapshot built this way needs no changes (EmbedderSnapshotData / Context::FromSnapshot as today).

Contract, documented on the field: the blob is a plain V8 startup blob (what mksnapshot emits), must outlive the setup, and the result loads only into a process on the same RO lineage - the rule V8 already imposes.

embedtest gets --create-v8-startup-blob <file> (a SnapshotCreator with just a default context) and --embedder-snapshot-base-blob <file>; the new test builds a plain blob, builds the embedder snapshot on top of it, and round-trips argv through the result like test-embedding-snapshot-as-file does.

Tests: new test-embedding-snapshot-base-blob.js; embedding suite and cctest pass. Locally I also generated node_snapshot.cc with node_mksnapshot --v8-snapshot-blob=<plain blob>, linked node against it and ran the snapshot/process/worker/vm/bootstrap suites on that binary: all pass.

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Disclosure: the code, test and this description were written by Claude Code, directed and reviewed by @codebytere.

[nodejs-github-bot]

Review requested:

• @nodejs/startup

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.12%. Comparing base (30bff4a) to head (99bfd98).
⚠️ Report is 36 commits behind head on main.

Files with missing lines	Patch %	Lines
src/api/embed_helpers.cc	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65374      +/-   ##
==========================================
- Coverage   90.13%   90.12%   -0.02%     
==========================================
  Files         752      752              
  Lines      251568   251865     +297     
  Branches    47270    47364      +94     
==========================================
+ Hits       226759   226988     +229     
- Misses      16168    16186      +18     
- Partials     8641     8691      +50     

Files with missing lines	Coverage Δ
src/node.h	92.45% <100.00%> (+0.14%)	⬆️
src/api/embed_helpers.cc	75.00% <66.66%> (-0.13%)	⬇️

... and 59 files with indirect coverage changes

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