iter embedding-abi-m1.1 (DONE 7/7): Embedding-ABI M1 — scalar AILang fn callable from C/Rust

Tasks 4-7 on the Boss-Repaired plan, completing the M1 iteration:

- Task 4: check-side export-signature gate — CheckError variants
  ExportNonScalarSignature / ExportHasEffects (codes
  export-non-scalar-signature / export-has-effects), code() arms,
  check_fn gate (params->ret->effects, first-violation-wins,
  unconditional on f.export.is_some()). The clause-3 discriminator
  in code: effectful/non-scalar exports fail to typecheck. 6/6.
- Task 5: codegen Target::StaticLib — Target enum, threaded param,
  @main/MissingEntryMain gated behind Target::Executable, one
  external @<sym> forwarder per export to @ail_<module>_<fn>
  (fn_scalar_sig/llvm_scalar). In-source missing_entry_main_is_error
  byte-unchanged. Lowering pin 2/2.
- Task 6: CLI ail build --emit=staticlib — clap field, Cmd::Build
  branch, build_staticlib (verbatim build_to prefix + has_export
  guard + two-archive clang -c/ar rcs tail) + run_cmd. CLI pin 2/2.
- Task 7: C-host E2E coherent-stop proof (cc links
  libembed_backtest_step.a + libailang_rt.a, backtest_step(0,3)=9 /
  (9,4)=25, s==25, exit 0) + DESIGN.md §"Embedding ABI (M1)"
  (scalar C ABI, provisional until M3). E2E pin 1/1.

Independent Boss verification: E2E 1/1, gate 6/6, full workspace
0 failures, roundtrip_cli + design_schema_drift + spec_drift green,
Invariant 1 holds (no new dep in core/codegen/runtime). 3 behaviour-
neutral plan pseudo-vs-reality concerns in the journal. Default-exe
codegen/runtime path byte-unchanged (check.py firing = tracked-P2
known-noise, audit-adjudicated). INDEX line added.

Completes the M1 iteration (impl). Milestone-close audit + fieldtest
(surface-touching) are the next pipeline steps.

examples/embed_export_float_ok.ail

@@ -0,0 +1,10 @@
+(module embed_export_float_ok
+  (fn scale
+    (export "embed_scale")
+    (type
+      (fn-type
+        (params (con Float) (con Float))
+        (ret (con Float))))
+    (params a b)
+    (body
+      (app * a b))))