v0.24.0

feat(http): surface init-request metadata to a producer's first process()

Over HTTP a producer stream's first turn runs inside the /init request, so unlike the pipe transport the worker never saw a real first tick. _run_http_producer_init now delivers the init request's Arrow metadata to the producer's first process() call.

This enables the VGI extension's HTTP result-cache conditional revalidation (304 / not_modified): the vgi.cache.if_none_match validator now reaches the worker before it produces. Also fixes the general gap where HTTP producers couldn't observe any first-exchange metadata (e.g. vgi_pushdown_filters).

Full suite green (1198 passed), mypy clean.