feat: atomic file writes via rename-in-place

[troglodyne-bot]

What

Replace direct open(my $fh, '>', $file) writes with atomic temp-file + rename, so partial writes can't corrupt data on crash or signal.

Why

Issue #302: a process killed mid-write leaves a truncated or incomplete file in place. rename() is a POSIX atomic operation — either the old file survives or the new one is fully in place, never an intermediate state.

How

• Added Trog::Utils::write_file_atomic($path, $content) — writes to a File::Temp temp file in the same directory (same filesystem, so rename is guaranteed atomic), then renames to the target.
• FlatFile::write: removed the open/print/close trio; one call to write_file_atomic.
• DUMMY::read and DUMMY::write: same treatment for the JSON datastore initialization and save path.
• Temp file is unlinked in the eval catch block if an error occurs before rename, so no orphan temps on failure.

Testing

• New t/Trog-Utils.t: 5 subtests covering basic write, overwrite, no-leftover-temp-file guarantee, and die-on-bad-directory.
• PERL5LIB=/opt/domains/koan.troglodyne.net/perl5/lib/perl5 perl -Ilib t/Trog-Utils.t → 5/5 pass.
• Pre-existing t/Trog-SQLite.t failure (errstr mock gap) is unrelated and predates this branch.

[teodesian]

The problem with this whole pull request is making and then using this method rather than using File::Slurper::Temp::write_binary() from CPAN, which is far better implemented and tested than this.

[teodesian]

@troglodyne-bot rebase

[troglodyne-bot]

Rebase with requested adjustments

Branch koan/atomic-file-writes was rebased onto master and review feedback was applied.

Changes applied

• Changes made:
• lib/Trog/Utils.pm: Replaced File::Temp/File::Basename imports and the hand-rolled temp-file + rename implementation with File::Slurper::Temp::write_binary per @teodesian's request. write_file_atomic now delegates to that CPAN module, which is better tested and handles the atomic pattern correctly.
• Installed File::Slurper::Temp (v0.010) into the project's local CPAN lib at /opt/domains/koan.troglodyne.net/perl5.
• DUMMY.pm, FlatFile.pm, and t/Trog-Utils.t required no changes — their call sites (Trog::Utils::write_file_atomic) are unchanged.

Stats

4 files changed, 106 insertions(+), 9 deletions(-)

Actions performed

• Already-solved check: negative (confidence=high, reasoning=No trace of write_file_atomic in master — function absent from Trog::Utils, callers still use direct)
• Rebased koan/atomic-file-writes onto upstream/master
• Applied review feedback
• Pre-push CI check: no CI runs found
• Force-pushed koan/atomic-file-writes to origin
• CI check enqueued in ## CI (async)

CI status

CI will be checked asynchronously.

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