Test the workerd collector's memory, queue and retention bounds

[RhysSullivan]

Runs the built worker as a real workerd process with its inspector open.

• Sustained ingest (36,000 spans at up to 400/s over 90 s): buffers the collector's isolate holds stay within maxPending × maxBytes + 8 MiB. The V8 heap, sampled without forcing a collection, stays at or below 64 MiB. Live heap after a collection stays under 32 MiB. After the retention pass the store holds at most the configured spans and bytes.
• With only the size bound in reach and no retention pass during the load, the database file never exceeds the bound by more than one export, and the stored bytes end within it.
• With the queue full of exports whose bodies are still arriving, the next export gets 429 and an oversized one 413; both are counted in GET /api/ingest, and the queue frees once the stalled exports finish.
• Malformed exports get 400 and are counted as invalid. The counts are the same after the collector restarts on the same store.

Results of this test file on each collector revision:

revision	heap / buffers	file size bound	queue	loss counts
edca7f8 (base, #2)	fail: 294 MiB buffers vs 12	fail: 34 MB stored vs 4 MiB	fail: 200, nothing refused	fail: 500 for malformed
2f5bd17 (router only)	fail: heap 92.8 MiB vs 64	fail	fail (new counters)	fail
89fbfd3 (direct ingest)	pass	fail: 34 MB stored vs 4 MiB	fail (new counters)	fail
c9422c3 (#5 head)	pass	pass	pass	pass

bun run test on #5's head with these tests: 217 pass, 0 fail.