fs: preserve negative utimes timestamps

[guybedford]

This is the semver-major alternative to #64627, fixes #64597.

toUnixTimestamp() silently replaced negative numeric timestamps with the current time, while negative strings and Dates were passed through as pre-epoch times:

fs.utimesSync(p, -1, -1);              // mtime = now
fs.utimesSync(p, '-1', '-1');          // mtime = 1969-12-31T23:59:59Z
fs.utimesSync(p, new Date(-1000), ...) // mtime = 1969-12-31T23:59:59Z

Removing the clamp exposed that Windows reinterprets stat tv_sec as unsigned long, so that pre-epoch times read back as post-2038 times (via static_cast<unsigned long> in FillStatsArray). This was a Node.js decision from #43714 to work around libuv's 32-bit long tv_sec on Windows, which is what the test-fs-utimes-y2K38.js Windows overflow_mtime case covered. As discussed in #64627, this PR treats negative times as pre-epoch on all platforms, consistent with POSIX:

• toUnixTimestamp() no longer replaces negative numbers with the current time.
• Windows stat times are no longer cast to unsigned, so negative values are pre-epoch times rather than post-2038 times. As a result, times after 2038-01-19T03:14:07Z are no longer representable on Windows until libuv gains 64-bit time support.
• Docs updated for the atime / mtime rules and the Windows time range.

Test changes:

• test-fs-utimes.js now computes the expected mtime independently of fs._toUnixTimestamp() (previously the oracle was the code under test, so the negative case always passed), uses an absolute difference, and adds negative number and pre-epoch Date cases. These are skipped on AIX, which rejects pre-epoch times with EINVAL.
• test-fs-utimes-y2K38.js skips the Y2K38 round trip on Windows and drops the overflow_mtime case, keeping the Windows precision check.

A CITGM run is suggested per the discussion in #64627.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.43%. Comparing base (ae5a0f4) to head (7f80f9f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66634      +/-   ##
==========================================
- Coverage   92.78%   90.43%   -2.35%     
==========================================
  Files         422      791     +369     
  Lines      193692   276589   +82897     
  Branches    29881    53112   +23231     
==========================================
+ Hits       179718   250137   +70419     
- Misses      13645    16860    +3215     
- Partials      329     9592    +9263     

Files with missing lines	Coverage Δ
lib/internal/fs/utils.js	96.27% <ø> (+4.82%)	⬆️
src/node_file-inl.h	85.48% <ø> (ø)

... and 498 files with indirect coverage changes

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