iter embedding-abi-m2.tidy: '## Embedding ABI (M1)' -> '## Embedding ABI' lockstep rename (M2 audit doc-honesty fix)

Resolves the M2 milestone-close audit's only actionable drift item
(architect [medium] DESIGN.md:2266 stale (M1) header vs its now-M2
present-tense body + coupled [low] :2354 xref). Current-state-mirror:
the section describes the current embedding ABI (M1+M2), so the
milestone tag is internal history, not current state.

5 live-coupling sites renamed in lockstep, 3 files:
- docs/DESIGN.md:2266 header '## Embedding ABI (M1)' -> '## Embedding ABI'
- docs/DESIGN.md:2354 schema-block xref 'see §"Embedding ABI"'
- docs_honesty_pin.rs:134 test comment + :136 assert-message
- crates/ailang-core/specs/form_a.md:89 live forward-xref (Boss-added
  5th site: plan-recon-undercount countermeasure surfaced a live
  dangling-xref the architect's 3-site scope missed; folded in on
  the merits — a half-rename contradicts the tidy's own thesis)

docs_honesty_pin.rs:135 asserted substring (the rename-immune
DESIGN.md body sentence) byte-verbatim untouched. Committed
append-only history NOT falsified (Boss-verified diff = 3 content
files + journal + stats only). Pins green before AND after
(docs_honesty_pin 5/0, design_schema_drift 8/0). Zero
language/checker/codegen/schema change.

docs/DESIGN.md

@@ -2263,7 +2263,7 @@ overlay shape. The asymmetry between the check side and the mono
 side is by design and is pinned by
 `crates/ailang-check/tests/duplicate_ctor_pin.rs`.
 
-## Embedding ABI (M1)
+## Embedding ABI
 
 `ail build --emit=staticlib` compiles a module to a relocatable
 `lib<entry>.a` (program objects only) plus a separate
@@ -2351,7 +2351,7 @@ are real surface forms.
   "params": ["<id>"...],   // names bound in body, in type.params order
   "body": Term,
   "doc": "<optional string>",
-  "export": "<optional C symbol>", // omitted when absent (hash-stable when omitted); see §"Embedding ABI (M1)"
+  "export": "<optional C symbol>", // omitted when absent (hash-stable when omitted); see §"Embedding ABI"
   "suppress": [Suppress...] // omitted when empty
 }