fix(hir): anonymous export default function () {} — emit __default symbol

[proggeramlug]

Summary

• HIR lowerer dropped the body of export default function () { ... } (no name binding) entirely, so codegen never emitted perry_fn_<src>__default and consumers link-failed with Undefined symbols: _perry_fn_<src>__default. The __perry_wrap_perry_fn_<src>__default rename wrapper from uuid (v4) fails to compile: default export name not wired up under compilePackages #837 also had nothing to point at.
• Fix: when fn_expr.ident == None and the function has a body, synthesize an ast::FnDecl with ident default and run it through the same lower-and-export path that ExportDecl::Fn uses (lower_fn_decl → is_exported → func_defaults + exported_functions + Export::Named { local: "default", exported: "default" }). The HIR function name default flows through sanitize() unchanged, so the LLVM symbol is perry_fn_<src>__default — what consumers ask for. Since f.name == exported_name, the alias-wrapper loop is a no-op and the undefined-stub fallback at codegen.rs:2310 skips emission because the real symbol exists.
• Scope-narrow: only the fn_expr.ident == None branch changes. Named-default (export default function foo() {}) has a separate bug (body also dropped) that is left for a follow-up per the issue directive. Closes the remaining-1-symbol case from zod fails to compile under perry.compilePackages: cross-module class re-exports unresolved #836's closeout.

Test plan

• Regression test added: test-files/test_issue_anonymous_default_export.ts + test-files/test_issue_anonymous_default_export_pkg/{producer,consumer}.ts (producer exports an anonymous default fn returning 42; consumer imports and invokes it; expected output 42).
• Output matches node --experimental-strip-types byte-for-byte (42).
• Zod link error from zod fails to compile under perry.compilePackages: cross-module class re-exports unresolved #836's closeout is gone — canonical repro (perry.compilePackages: ["zod"], import { z } from "zod"; console.log(z.object({a:z.string()}).parse({a:'hi'}).a);) compiles + links successfully (runtime divergence remains, separate).
• Existing zod fails to compile under perry.compilePackages: cross-module class re-exports unresolved #836 regression test (test_issue_836_zod_class_reexports.ts) still passes.
• cargo build --release -p perry-runtime -p perry-stdlib -p perry clean.
• cargo test --release -p perry-hir -p perry-codegen clean.
• cargo test --release --workspace (with the usual UI-crate excludes) exits 0.

Refs #793, #836, #837.