ariadne: generic async URI resolver — (fetch uri) / (fetch-async uri) (§13.8)

[transfix]

Adds a scheme-agnostic async resolver so an ari program can load any registered URI scheme (file/state/cvc/http) without blocking the scheduler thread — the DSL surface for "all ari-generated URI requests async". Builds on PR-A (#474)'s park/future machinery.

(get-attr (fetch "state://sys.cfg?value") "body")   ; any scheme, async, no thread blocked
(await (fetch-async "file:///x.ari"))                ; a future you can await

What it does

launch_uri_fetch runs the synchronous cvc::ariadne::resolve() on a compute-pool worker (off the scheduler thread) and posts the marshalled result to a unique #-reply channel — the same launch shape as launch_http_fetch. Two verbs reuse PR-A's park/future primitives:

• (fetch-async URI [BASE]) → a future handle (await it, or msg-recv it).
• (fetch URI [BASE]) → transparent: self-parks and returns the dict directly.

Reply dict: { ok body(bytes) url error } (scheme-agnostic — no HTTP status/headers; use http-get for those). The register gate is split so (fetch*) needs no HTTP backend (works over any registered scheme, even in a curl-less build) while (http-get*) still require cvc::net::have_http_backend(). resolve() is app-free with its own barrier; the worker wraps it and always posts a dict, so a waiter never hangs.

Scoping (the honest answer to "all ari URI requests async")

The other ari-generated URI requests stay synchronous by design: import:/load:/include: resolve at load time, off the draw walk (roadmap §13.8 — sync is legal there), and scene source:{uri:} is a one-shot at scene setup (not per-frame). So nothing ari-generated blocks the render thread today; a DSL program that wants async loading uses (fetch). An async scene source:{uri:} realize (so a large remote asset doesn't block scene setup) is a natural follow-up (it needs an async resolve_to_file + temp-file lifetime across the hop).

Tests

ariadne_runtime_test (AriadneNetIntrinsics, offline): (fetch) transparently resolves a canned custom scheme with the bytes body threaded in; (await (fetch-async …)) resolves the future. Full ariadne_runtime_test 106, 0/20 flaky.

Next in the arc: render-pass park budget + safety (PR-C — residents currently run with no time budget).