fix: use process-unique temp names for session saves

[docxology]

save_json_to_path used target.with_extension("json.tmp") — a fixed name not unique per write. If two processes (e.g. a herdr server and client, or two server instances) save concurrently, they clobber each other's in-progress temp file, and one can rename() a file the other is still writing, producing a truncated/garbled session.json that fails to parse on next load (silent total session loss).

Fix: use a process-unique temp file name incorporating PID, an atomic counter, and nanosecond timestamp: .{file_name}.{pid}.{seq}.{nanos}.tmp. This ensures no two writers collide on the same temp path.

Regression tests: unique_tmp_path_is_unique_per_call and save_to_paths_leaves_no_tmp_files_behind verify uniqueness and cleanup.

cargo fmt --check: clean
cargo test --locked persist: 9 passed, 0 failed

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📝 Walkthrough

Walkthrough

Session persistence now creates unique temporary paths for atomic JSON writes. The paths remain beside the target file and include process and sequence identifiers. Tests verify path properties and cleanup after session and history saves.

Changes

Persistence temporary paths

Layer / File(s)	Summary
Unique temporary path generation src/persist/io.rs	Adds a process-local counter and timestamp-based temporary filename generation beside the target file.
Atomic save integration and validation src/persist/io.rs	Uses unique temporary paths for atomic writes. Tests verify uniqueness, .tmp suffixes, same-directory placement, and cleanup.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• herdrdev/herdr#2335: Both changes update temporary-file handling in src/persist/io.rs for session and history persistence.

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[docxology]

Closing — the maintainer requested no unsolicited fix PRs. These findings are documented in case they're useful for future issue reports.