Observation registration is per-webview, so independent callers in one window collide

[jjhafer]

ObserverRegistrations keys registrations by webview label
(src/subscriptions.rs:257), so two independent frontend modules in the same
window that each call observe() collapse into a single registration. The first
one to call unobserve() hits remaining == 0 and triggers full teardown
(src/commands.rs:900-904), aborting the other module's subscriptions — #54's
symptom, in-window.

PR #55 documents this as a caller obligation (a window needs a single owner of the
observe/unobserve pair) and #55's subscribe() registration check does not help:
both modules share the label, so both pass it.

Note on the obvious fix: adding webview_label to ActiveSubscription and
scoping teardown by it does not fix this case. Both modules share the label, so
a label-scoped remove_for_db aborts the identical set — and teardown also calls
disable_observation(), which sets observer = None
(crates/sqlx-sqlite-toolkit/src/wrapper.rs:449-451) and drops the broker, so
every stream ends regardless. Scoping would leave the second module's dead entry
behind until self-reap, i.e. strictly less cleanup for no benefit.

Possible approaches

• A per-caller registration token returned by observe() and passed to
  unobserve(), replacing label-keyed refcounting.
• A JS-side refcount in guest-js. Needs a module-level registry, because
  Database.get() returns a fresh instance per call
  (guest-js/index.ts:861-862), so per-instance state would not be shared.

Both are public-API changes, hence out of scope for #55.

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Split out of the code review on #55, where it was validated as real but out of scope for that PR. See the review threads there for the supporting analysis.

Issue filed by AI model Claude