fix(hir): #1070 — instanceof narrowing breaks property read in fastify error handler

[proggeramlug]

Closes #1070.

Root cause

pre_scan_fastify_handler_params (in crates/perry-hir/src/lower_patterns.rs) registered the first arrow param of every recognized fastify handler call as a ("fastify", "Request") native instance — including app.setErrorHandler(async (err, req, reply) => …), whose first param is the thrown value, not a Request.

With err tagged as Request, any user-field read like err.problem lowered to a NativeMethodCall { module: "fastify", method: "problem", … }. problem isn't in the Request method dispatch table, so the lower_native_method_call fall-through (crates/perry-codegen/src/lower_call/native.rs:2297) returned double_literal(0.0) — the access printed as 0.

(err as MyError).problem worked because expr_member.rs's native-instance shape check only matches member.obj == Expr::Ident; a TsAs cast bypassed it and went through generic property dispatch — which is why the bug report's outside-the-if access printed the full object but the inside-the-narrow bare err.problem access printed 0.

Fix

Skip the err param for setErrorHandler so only params[1]=request and params[2]=reply get the native-instance tags. err.<field> then falls through to the generic property dispatch that reads the actual class-instance field at runtime.

Verification

Reproducer from the issue:

[A] outside if — problem: {"title":"Bad Request","status":400}
[A] inside if — problem: {"title":"Bad Request","status":400}   ← pre-fix: 0
[A] const p: {"title":"Bad Request","status":400}                ← pre-fix: 0
[B] no-narrow as-cast — problem: {"title":"Bad Request","status":400}

Regression coverage added in test_fastify_integration.ts + run_fastify_tests.sh:

• POST /throw-problem throws a ProblemError instance from a route
• setErrorHandler narrows via instanceof and reads err.problem.{status,title}
• HTTP 400 with {"title":"Bad Request","status":400} body

All 12 fastify integration tests pass (10 pre-existing + 2 new for #1070).

No version bump or changelog change — maintainer handles that at merge time.

Test plan

• Reproduced the bug locally with the issue's exact repro
• Verified the fix restores correct field reads
• All cargo test -p perry-hir and cargo test -p perry-codegen pass
• scripts/run_fastify_tests.sh: 12/12 pass (was 10/10 pre-fix + 2 new for instanceof narrowing breaks property access in Fastify async error handler #1070)