fix(backend): give each report file a name of its own (closes #61)

Report files were named by a millisecond timestamp and created with
O_EXCL, so two flushes in the same millisecond, such as a flush for
size and the final flush at shutdown, got the same name and the second
failed, losing its reports. Each name now carries a number after the
timestamp that goes up by one for each file the server starts to
write, so names still sort by time and never repeat within a run; a
failed write leaves a gap in the numbers. The buffer reads the time
through a clock the new test stops, so its two flushes share one
timestamp on every run; the 1 ms pauses earlier tests used to dodge
the collision are gone.

Model: opus-5-5

TODO.md

@@ -24,10 +24,11 @@ latest run passes.
 # Completed Steps
 
 - 2026-09-29: report file names can no longer collide (issue #61): each is
-  `reports-<timestamp>-<number>.jsonl.zst`, where the number counts the files
-  written since the server started, so two flushes in the same millisecond, such
-  as a flush for size and the final flush at shutdown, each get a file of their
-  own instead of the second one failing
+  `reports-<timestamp>-<number>.jsonl.zst`, where the number goes up by one for
+  each file the server starts to write, so two flushes in the same millisecond,
+  such as a flush for size and the final flush at shutdown, each get a file of
+  their own instead of the second one failing. A failed write leaves a gap in
+  the numbers.
 - 2026-09-29: nginx listens on `PORT` (issue #26), 8080 when unset or empty: the
   nginx image renders `nginx.conf` as a template at container start, filling in
   `PORT` and no other variable. `bin/entrypoint.sh` refuses to start when `PORT`

backend/README.md

@@ -104,10 +104,11 @@ which `netwatch` owns.
 
 Reports are written as `reports-<timestamp>-<number>.jsonl.zst` files in
 `DATA_DIR`. The timestamp is in UTC to the millisecond, so the names sort by
-time; the number counts the files the server has written since it started, so
-two files written in the same millisecond still get different names. Each file
-contains one JSON object per line, compressed with zstd. Files are created with
-`O_EXCL` to prevent overwrites.
+time. The number starts at 1 when the server starts and goes up by one for each
+file the server starts to write, so two files written in the same millisecond
+still get different names, and a failed write leaves a gap in the numbers. Each
+file contains one JSON object per line, compressed with zstd. Files are created
+with `O_EXCL` to prevent overwrites.
 
 ### Report limits