fs: add openAsBlobSync

[greenheadHQ]

This adds fs.openAsBlobSync(path[, options]) as a synchronous counterpart to fs.openAsBlob(). For real-file paths, it returns the existing file-backed Blob directly while leaving the Promise-returning API unchanged; reading the Blob's contents remains lazy and asynchronous.

This follows up on #65462. Compared with #49759, the change is additive: it does not modify fs.openAsBlob() or add an fs.promises alias.

Documentation is included. Tests cover supported path types and validation, file mutation detection, Permission Model denial, VFS mount ownership, and verify that fs.openAsBlob() still returns a Promise.

Disclosure: I used codex for research and some implementation work. I reviewed and thoroughly tested the changes myself.

Refs: #65462
Refs: #49759

[nodejs-github-bot]

Review requested:

• @nodejs/security-wg
• @nodejs/startup

[codecov]

Codecov Report

❌ Patch coverage is 89.65517% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.07%. Comparing base (8fe4b64) to head (90fe976).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/bootstrap/switches/is_main_thread.js	0.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #65644   +/-   ##
=======================================
  Coverage   90.07%   90.07%           
=======================================
  Files         754      754           
  Lines      256317   256343   +26     
  Branches    48476    48483    +7     
=======================================
+ Hits       230871   230904   +33     
  Misses      16563    16563           
+ Partials     8883     8876    -7     

Files with missing lines	Coverage Δ
lib/fs.js	98.44% <100.00%> (+<0.01%)	⬆️
lib/internal/vfs/setup.js	87.36% <100.00%> (+0.02%)	⬆️
lib/internal/bootstrap/switches/is_main_thread.js	90.66% <0.00%> (ø)

... and 32 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/76748/

[greenheadHQ]

Rebased onto the latest main to resolve a conflict with the recently landed #63653. The only manual conflict resolution was in the VFS test, which now uses the mount point returned by vfs.mount() and the new reserved VFS namespace.

The API scope remains unchanged. The focused fs, VFS, and Permission Model tests, along with lint and documentation tests, pass.