fs: expose frsize field in statfs results

[juicecultus]

Description

fs.statfs() exposes bsize (optimal I/O block size) but not frsize (fundamental filesystem block size). Per POSIX, block counts (blocks, bfree, bavail) are in units of frsize, not bsize. libuv already reads f_frsize from the kernel and stores it in uv_statfs_t — it was simply never mapped to JavaScript.

On most native filesystems bsize == frsize, so the omission was harmless. However, on FUSE mounts (e.g. Docker Desktop on macOS with VirtioFS or gRPC FUSE), they can diverge by orders of magnitude:

Field	Value	* blocks result
bsize	2,097,152 (2 MiB)	1.82 PiB ❌
frsize	16,384 (16 KiB)	14.9 TiB ✅

This causes any application computing disk space as bsize * blocks to report wildly inflated values. Real-world example: Immich photo manager shows "602 TiB of 1.8 PiB" on a 15 TB disk when running in Docker on macOS.

Changes

Adds the frsize property to the StatFs object returned by fs.statfs(), fs.statfsSync(), and fsPromises.statfs(), in both normal and bigint modes.

Files changed:

• src/node_file.h — add kFrSize to FsStatFsOffset enum
• src/node_file-inl.h — map s->f_frsize in FillStatFsArray()
• lib/internal/fs/utils.js — add frsize to StatFs class and binding
• doc/api/fs.md — document the new field
• test/parallel/test-fs-statfs.js — include frsize in property checks
• test/parallel/test-fs-promises.js — include frsize in type assertions

Notes

• On Linux, libuv sets f_frsize from the kernel's statfs.f_frsize
• On non-Linux (macOS, etc.), libuv falls back to f_frsize = f_bsize, so this is always populated
• This is a semver-minor addition (new property on existing object)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.71%. Comparing base (bdf75a6) to head (cd87e8a).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #62495   +/-   ##
=======================================
  Coverage   89.71%   89.71%           
=======================================
  Files         692      692           
  Lines      213988   214042   +54     
  Branches    41054    41062    +8     
=======================================
+ Hits       191976   192026   +50     
- Misses      14086    14090    +4     
  Partials     7926     7926           

Files with missing lines	Coverage Δ
lib/internal/fs/utils.js	99.68% <100.00%> (+<0.01%)	⬆️
src/node_file-inl.h	87.71% <100.00%> (+0.05%)	⬆️
src/node_file.h	78.43% <ø> (ø)

... and 35 files with indirect coverage changes

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