Clear the error channel when a fallible Void function falls through

Fixes #436.

Problem

A fallible Void function that ends without return left the last value in rax (x0 on arm64). Callers read rax >> 32 as the error code, so any leftover value with non-zero high 32 bits looked like a raised error. The explicit valueless return already clears it (emit_elf64.c:3899), but the fall-through path didn't.

Fix

Emit the same clear on the fall-through path when fun->raises, in emit_elf64.c, emit_macho_x64.c and emit_macho64.c. The other backends reject check/raise today, so they can't reach this path.

conformance/native/pass/fallible-void-fallthrough.0 covers an empty body, a local, a parameter (2^32 and -1), a mutref field store, an inferred error set, and a genuine raise that must still propagate. It's registered with assertDirectRuntimeRequired. assertDirectRuntimeOrUnsupported and run_native_or_gap both treat a non-zero exit as a backend gap, so neither could have caught this.

The new lines push emit_elf64.c and emit_macho_x64.c past their compiler-metrics line budgets, so those are bumped to the new counts.

Verification

• linux-x64 (WSL). Running the fixture through the assertDirectRuntimeRequired build/run sequence exits 1 with no output on 7e1a64d, and prints fallible void fallthrough ok with this change. Each case also fails on its own without the fix.
• Existing programs with fallible helpers (examples/fallibility, direct-raises-basic, direct-rescue-basic, checked-fallible-wrapper, world-output-helper) behave the same. No size budget or snapshot moves.
• compiler-metrics.mts and graph-input-policy.mts pass.
• Mach-O x86-64 and arm64 checked from the emitted bytes only. I don't have a Mac to run them.
• I couldn't run pnpm run conformance locally (no node 24 or pnpm here, and the musl runtime-linked builds need zig), so the full suite is down to CI.

Not in this PR. ELF only skips the fall-through epilogue after RETURN, while Mach-O also skips it after RAISE, so ELF emits a dead xor; leave; ret after a trailing raise. It's harmless, and I left it alone to keep this small.

🤖 Generated with Claude Code