fix(fastify): #1293 route (request as any).json()/.body through external-fastify handle dispatch

[proggeramlug]

Fixes #1293.

Root cause

The canonical Fastify body-parse pattern casts the receiver:

const body = (request as any).json();
if (!body) return reply.status(400).send({ error: "Invalid body" });

The as any cast erases the static type, so codegen lowers .json() to a generic dynamic dispatch (Call { callee: PropertyGet }) instead of a NativeMethodCall { module: "fastify" } — confirmed via --print-hir. At runtime that lands in perry-stdlib's js_handle_method_dispatch / js_handle_property_dispatch.

Those dispatchers probe only perry-stdlib's own handle registry (with_handle::<crate::fastify::FastifyContext>). But the well-known flip routes fastify → perry-ext-fastify, whose FastifyContext lives in perry-ffi's separate registry (the two are documented as disjoint integer spaces). The probe missed, the call fell through, and:

• (request as any).json() returned a bare NaN (typeof "number", String(j) = "NaN")
• (request as any).body returned undefined

…silently 400-ing every POST that used the Fetch-style accessor. The if(!body)-vs-typeof distinction in the report was incidental — the value was wrong in all cases on current main; the typed request.json() (no cast) path from #1240 was the only one that worked.

Fix

Mirrors the established external-net-pump pattern:

• perry-ext-fastify exports js_ext_fastify_is_context_handle — a membership probe over its own perry-ffi registry.
• perry-stdlib dispatch grows external-fastify-pump arms (gated not(http-server)) for both method and property dispatch that consult the probe and forward to the linked js_fastify_* exports — full parity with the bundled http-server arms (json, body, method, url, params, headers, send, status, code, header, type / query, rawBody, text, user).

Validation

Exact issue repro now matches the "Expected" output — all three routes return 200 with the parsed body:

=== no-anchor ===   {"where":"no-anchor","body":{"hello":"world"}}
=== with-anchor === {"where":"with-anchor","body":{"hello":"world"}}
=== use-body ===    {"where":"use-body","body":{"hello":"world"}}

• New wire-level harness test-files/run_test_issue_1293.sh (typed json(), (as any).json(), (as any).body) — PASS
• Existing run_test_issue_1240.sh (typed path) — PASS (no regression)
• New perry-ext-fastify unit test for the membership probe — PASS
• cargo test -p perry-ext-fastify (12) + cargo test -p perry-stdlib (74) — PASS
• cargo fmt --all -- --check — clean

Note for merge: version bump + CHANGELOG entry intentionally omitted (folded in at merge per repo convention).