Frontend half of #408 — wires flow.js's terminal subscribe URL to
the kinds-filter damocles shipped in #499. Only the four kinds
flow.js actually handles get sent on the wire:
- sent / delivered → broker terminal renderer
- container_state_changed / container_removed → local autocomplete
cache (flowContainers Map used by the @-mention composer)
Everything else (approval_*, question_*, transient_*, tombstones_*,
meta_*, rebuild_queue_*) is now dropped server-side before the
JSON-serialise, instead of being deserialised + dispatched to
flow.js's no-op `_default: () => {}` per frame.
Tabs.js keeps the unfiltered `/dashboard/stream` subscribe since
it routes every mutation kind into its derived stores.
## SharedWorker note
`openStream` keys subscriptions by full URL (#448, #453). The
filtered URL is therefore its own upstream connection to
hive-c0re — when both /index.html and /flow.html are open at the
same time the worker holds two upstreams (one filtered, one full)
instead of today's single shared upstream. The trade is small
per-frame wire-byte + serialise savings vs +1 backend connection
in the both-open case. The flow-page-only case is a pure win.
Acceptable per #408's intent ("split flow messages from main
endpoint").
Round-trip note: if the backend's `kind_tag` strings ever drift
from what's listed here, the affected kind silently never matches
and flow loses that event class. damocles's #499 added a serde
round-trip test against `kind_tag` to guard the backend side;
the frontend mirrors the spelling exactly from the PR body.
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