fix(storage/local): write files atomically

[marino39]

Problem

LocalFS.Create (go-storage's local FS) truncates the target and writes it in place. For files rewritten in place, most importantly the .fileIndex:

• Readers race the writer. A reader loading the index while the writer saves it can read a partial file and fail with failed to load file index: unexpected EOF.
• A crash mid-save corrupts the index for good. We hit this in a chainsight test run: a reader's partial read failed the process, the ethlog writer exited mid-save, and its 380 KB index was left with only the first 40,330 of 76,265 entries. It had to be rebuilt by hand from the data files.

Change

storage/local.LocalFS now implements Create itself:

• Temp file: the data goes to a hidden temporary file in the target's directory, .<name>.tmp-*.
• Atomic publish: Close syncs that file and renames it over the target. Readers see the previous file or the complete new one, never part of one. A crash leaves the previous file intact.
• Failed writes discarded: if any Write failed, Close removes the temporary file, keeps the previous one and returns the error.
• Unchanged behaviour: files are created 0644, as os.Create gives them under the usual umask, and WriterOptions mod/creation times are still applied.

Only the local FS changes. GCS uploads are already atomic.

Tests

• TestFileIndex_ConcurrentSaveAndLoad: saves a 20k-entry index 30 times while another goroutine loads it. On master it fails with unexpected EOF within milliseconds; with this change it passes.
• storage/local unit tests:
  • nothing is visible at the path before Close;
  • the previous file stays readable until Close;
  • a failed write never replaces the file;
  • no temporary files are left behind;
  • file mode is 0644.

go test ./... passes.

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