sea: mount bundled assets as a virtual file system

[mcollina]

This adds the SEA integration for the virtual file system landed in #63653: setting "useVfs": true in the SEA configuration mounts the bundled assets as a read-only VFS and runs the injected main script from inside the mount.

Since a VFS never shadows the real file system and its mount point is reserved and chosen at runtime, there is no fixed /sea path. Instead, the main script is placed at the root of the mount and executed from there, so:

• __filename and __dirname point inside the virtual file system;
• assets are reached through __dirname-relative paths with regular node:fs APIs;
• relative require() calls and node_modules lookups resolve against the bundled assets, confined to the mount by the module loader integration.

Implementation notes:

• The main script is not duplicated into the assets at build time; its source already lives in the SEA blob and is injected into the provider at runtime.
• SEAProvider is read-only and lazy: asset content stays in the executable's SEA blob and is copied into JS memory only when a file is opened, with sizes cached for stat.
• The implicit SEA mount does not emit the VirtualFileSystem experimental warning; the SEA warning already covers it.
• ESM entry points ("mainFormat": "module") are supported: the ESM main is loaded from inside the mount through the ESM loader, so import.meta.url/filename/dirname reflect the mount and static imports, dynamic import(), and bare specifier lookups resolve against the bundled assets.
• "useVfs" is rejected together with "useSnapshot" and "useCodeCache".

---

This PR was prepared with the help of AI. I've reviewed all changes myself.

[nodejs-github-bot]

Review requested:

• @nodejs/single-executable

[codecov]

Codecov Report

❌ Patch coverage is 99.40476% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.07%. Comparing base (4d9cb71) to head (6ee9698).
⚠️ Report is 22 commits behind head on main.

Files with missing lines	Patch %	Lines
src/node_sea.cc	93.47%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65675      +/-   ##
==========================================
+ Coverage   90.05%   90.07%   +0.02%     
==========================================
  Files         754      756       +2     
  Lines      256308   256897     +589     
  Branches    48464    48563      +99     
==========================================
+ Hits       230817   231405     +588     
- Misses      16593    16615      +22     
+ Partials     8898     8877      -21     

Files with missing lines	Coverage Δ
lib/internal/main/embedding.js	91.89% <100.00%> (+2.92%)	⬆️
lib/internal/vfs/providers/sea.js	100.00% <100.00%> (ø)
lib/internal/vfs/sea.js	100.00% <100.00%> (ø)
src/node_sea.h	100.00% <ø> (ø)
src/node_sea.cc	89.77% <93.47%> (+0.27%)	⬆️

... and 46 files with indirect coverage changes

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