fix: #748 — fastify wait_for_promise condvar wait + Rejected→500 branch

[proggeramlug]

Summary

• In both crates/perry-ext-fastify/src/server.rs::wait_for_promise() and crates/perry-stdlib/src/fastify/server.rs::wait_for_promise(), the dispatcher polled js_promise_state 10000 × 100 µs (≈1 s) and then returned regardless of state. After return, js_promise_value(ptr) (initialized to 0.0 in Promise::new) gave Rust f64 0.0 for any Pending or Rejected promise. The dispatcher's build_response_body then serialized that as the literal byte 0x30 ("0") with the default status_code = 200 — matching Native route returns body 0 + HTTP 200 after sequence of @perryts/mysql writes; first INSERT commits, rest silently no-op, explicit return dropped #748's wire symptom exactly. The orphaned chain was also abandoned: once the dispatcher returned, microtasks stopped pumping, so every queued INSERT after the timeout silently no-op'd.
• Replaces the polling/sleep loop with a condvar-based wait via js_wait_for_event — the same primitive the codegen-emitted await body already uses. No fixed iteration limit; wakes the moment any stdlib worker calls js_notify_main_thread. Drives timer ticks + stdlib pump + microtasks every iteration.
• Adds a Rejected branch: after wait_for_promise returns, branch on js_promise_state and respond HTTP 500 with a { "error": <reason> } JSON envelope instead of letting js_promise_value return 0.0 and masquerade as a successful response.

Closes #748.

Caveat: I could not exactly reproduce the user's "first INSERT commits, rest silently no-op" symptom in a synthetic repro without @perryts/mysql linkage. The wait_for_promise 1 s timeout path is the only place I found where js_promise_value(0.0_initial) reaches the response writer, and the wire byte 0x30 + HTTP 200 + dropped explicit return + abandoned subsequent awaits all match. Worth re-running skelpo-shop-admin against this build to confirm; if it still repros, PERRY_MT_PROFILE=1 will narrow the stall point.

Test plan

• cargo build --release -p perry-ext-fastify -p perry-stdlib green
• /tmp/repro748_fastify.ts — Fastify handler with 8×300 ms sequential awaits (2.4 s total). Pre-fix: would return HTTP 200 body 0 once the chain exceeded 1 s. Post-fix: returns full JSON body + HTTP 201 after ≈2.46 s.
• Existing test_fastify_integration.ts still passes; 10 fastify-ext unit tests + 22 fastify-stdlib unit tests all pass.
• Gap suite unchanged: 33 pass / 2 pre-existing failures (test_gap_console_methods, test_gap_regexp_advanced).