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feat(web): multi-source single-flight — named flight-data sources

The single-flight channel assumed exactly one data-owning integration:
one collectFlightData hook on the server, one subscribeFlightData
consumer on the client, later registrations displacing earlier ones. An
app running two caches (a router's route data, a query library's
client) had no way to refresh both from one mutation response.

The channel now multiplexes named sources over the same round trip.
registerFlightDataSource(id, hook) registers a server collector
additively next to the unnamed slot (which stays the data-owning
router's); subscribeFlightData(id, consumer) is its client half. The
request-leg X-Single-Flight header carries the subscribed source ids —
the server only runs collectors the client can consume — and the
response leg echoes the ids actually folded, making the payload shape
self-describing: named sources produce the keyed envelope
{ [source]: slice }, each slice delivered to its consumer before the
mutation's promise resolves.

Wire-compatible in every cross-version pairing: a lone unnamed
registration still sends and echoes the literal "true" with the raw
payload shape, byte-identical to the previous protocol, and
unrecognized hand-tagged opt-in values still reach the unnamed hook.
Existing integrations (Solid Router, TanStack Solid Start) work
unchanged; the keyed envelope only materializes when a named source
registers on both ends.

Co-authored-by: Cursor <cursoragent@cursor.com>

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