Address robot's review on PR #5:
- [Blocking] 'goon_referers' is a host-keyed GLOBAL store and must outlive
its capture tab; it was wrongly added to the per-tab onRemoved cleanup, so
closing any tab wiped referers captured by still-open source tabs (-> 412).
It is no longer removed on tab close. Stale referers are now pruned
opportunistically in persistReferer via a per-host timestamp + TTL.
- [Should-fix] the DNR referer rule used a single shared REFERER_RULE_ID with
a non-atomic check+update, so concurrent proxy fetches for two hosts could
interleave and leave the wrong host's referer live. Each host now gets its
own stable rule id (refererRuleIdFor) and the install+record is serialized
per host via an async lock (refererRuleLocks).
- [Coverage] add __tests__/referer_proxy.test.mjs exercising the new
referer-capture / proxy-host / onRemoved paths: closing a tab preserves
goon_referers, two hosts proxied concurrently keep distinct per-host rules,
and same-host proxy is idempotent.