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

@@ -27,9 +27,8 @@ latest run passes.
   `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 uses up its
-  number, leaving a gap if the file could not be created and otherwise a file
-  under that number that may be incomplete.
+  their own instead of the second one failing. A failed write leaves a gap in
+  the numbers.
 - 2026-09-29: ready to run under upaas (issue #59): the image has a
   `HEALTHCHECK` that requests `/.well-known/healthcheck` through nginx on the
   port from `PORT`. The backend no longer reads a bad `PORT` as 0 or a bad

backend/README.md

@@ -110,10 +110,9 @@ 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 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. A failed write uses up its number, leaving a gap in
-the numbers if the file could not be created and otherwise a file under that
-number that may be incomplete. Each file contains one JSON object per line,
-compressed with zstd. Files are created with `O_EXCL` to prevent overwrites.
+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