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 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. 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

@@ -23,6 +23,13 @@ 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 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.
 - 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