test: reproduce late map subscription sync gaps

[tmsteph]

Summary

Adds a PANIC regression test for an intermittent synchronization gap when a browser subscribes to an existing dynamic index with .map().on() after another peer has populated it through a relay.

The test deliberately does not reload Bob or issue a second explicit request. It expects every indexed record to arrive from the initial subscription.

Scenario

1. Start one Node relay.
2. Alice connects and writes item records plus dynamic index fields.
3. Bob initializes only after Alice's writes are acknowledged.
4. Bob subscribes to the index using .map().on().
5. Assert that Bob discovers every indexed record within 30 seconds.

Reproduction results

Using the repository's current master build:

• 100 records: passed 5/5 runs
• 200 records: passed
• 250 records: failed in repeated runs; Bob received 86/250 once and 0/250 once
• 1,000 records: Alice's write acknowledgements timed out after two minutes and Bob received 0/1,000

For comparison, the npm/CDN release used by the application where this behavior was first observed (0.2020.1241) passed 250/250 twice in approximately 5.6–6.8 seconds.

These results suggest the behavior is load-sensitive and may differ by build/version. This draft contributes the reproduction harness first; it does not propose a core fix yet.

Running the test

npx mocha test/panic/map-late-subscribe.js

The item count and browser/GUN paths can be overridden:

PANIC_ITEMS=250 \
PANIC_CHROMIUM_EXECUTABLE=/path/to/chromium \
npx mocha test/panic/map-late-subscribe.js

The test also accepts PANIC_GUN_ROOT and PANIC_GUN_BROWSER for cross-version comparisons.

Related: #1406, #1404, #1316, #1330.

[amark]

@tmsteph you're a hero (also, please DM / text me cuz GitHub doesn't notify me), flying back from few international travels now.

With a replicable failure, I'm hoping an AI can pinpoint in a trace/log the gap. My hunch is my special CPU breathing loops (probably not the poll/turn etc. utility itself) where it's queuing/dequeuing a get ack / reply and maybe the queue gets mutated while it breathes & it then misses it.