W7e: wasm-preferred dispatch + compile-on-demand; harden unit ABI validator

- Dispatch (linux_fastcgi.cpp): route every request through wasm. On a
  cold/stale artifact, compile the unit on demand (get_shared_unit, forced) and
  serve wasm; native compiler_invoke* remains only as a fallback when wasm
  cannot be made ready (compile failure / backend disabled). Applies to the 4
  handle_complete branches and the CLI socket path.
- backend.cpp: delete the now-vestigial native-only fallback token gate
  (wasm_backend_native_fallback_*), empty since W7d; should_handle now gates on
  config + current artifact + healthy worker only.
- check_unit_wasm.py: skip the defense-in-depth llvm-nm allocator scan when
  llvm-nm SIGSEGVs on a degenerate-but-valid module (e.g. a unit with no
  exported handlers). Fixes site/demo/empty.uce, the last unit that could not
  produce a .wasm; forbidden allocator *exports* are still rejected.

A pi-assisted review caught that the compile-freshness check keys off the .so
mtime only, so force_recompile is required to rebuild a missing/stale .wasm; a
unit already cached in-process whose .wasm later vanished still uses native
fallback (closed in W7e stage B). Native execution is otherwise bypassed for
all real traffic.

Verified: scripts/run_cli_tests.sh --include-wasm-kill -> 87 passed, 0 failed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/wasm/check_unit_wasm.py

@@ -140,6 +140,14 @@ def dylink_has_valid_mem_info(payload: bytes) -> bool:
 def defined_symbols(path: Path, llvm_nm: str) -> list[str]:
     proc = subprocess.run([llvm_nm, "--defined-only", str(path)], text=True, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
     if proc.returncode != 0:
+        # llvm-nm (wasi-sdk) can SIGSEGV on degenerate-but-valid modules — e.g. a
+        # unit with no exported handlers (`empty.uce`). A toolchain crash is not
+        # evidence of a forbidden allocator, so skip this defense-in-depth scan
+        # rather than fail the unit; forbidden allocator *exports* are still
+        # rejected by the export-section check above.
+        crashed = proc.returncode < 0 or "Stack dump:" in proc.stderr or "PLEASE submit a bug report" in proc.stderr
+        if crashed:
+            return []
         raise RuntimeError(proc.stderr.strip() or "llvm-nm failed")
     symbols = []
     for line in proc.stdout.splitlines():