ail: embed Form-A spec in merge-prose prompt (iter 20f)

Closes a design hole shipped in 20d: the merge-prose prompt instructed
the LLM to emit JSON-AST and gave a 12-line schema-essentials reminder.
JSON-AST is the canonical hashable artefact, not a writing surface; the
reminder was not a language spec. Foreign LLMs had no realistic shot.

20f makes two coupled changes:

1) The LLM now emits Form-A (the canonical authoring surface fixed by
   Decision 6). merge-prose loads the original via ailang_core::load_module
   and re-renders via ailang_surface::print before embedding (round-trip
   is a gating contract on the surface crate, so this is lossless). The
   user runs `ail parse foo.new.ailx` to recover JSON, then `ail check`.

2) crates/ailang-core/specs/form_a.md is the complete LLM-targeted
   Form-A specification — grammar, every term/pattern/type/def keyword,
   schema invariants, pitfall catalogue, four few-shot modules from
   examples/*.ailx. Exported as ailang_core::FORM_A_SPEC via include_str!
   and embedded verbatim in every merge-prose prompt.

Drift detection in crates/ailang-core/tests/spec_drift.rs: every variant
of Term, Pattern, Type, Def, Literal is reached via exhaustive `match`.
The arms are not the assertion — adding a new variant without updating
the match is a compile error before the test runs. Once matched, an
anchor string is asserted to appear in FORM_A_SPEC. 8 tests, all green.

The hand-written + mechanical-drift-test combo addresses the user's
"distance to code is too big" concern about a docs-only spec. Generator
overkill rejected: structural drift is mechanically caught, but prose
explanation, schema-invariant catalogue, pitfall list, and few-shot
corpus cannot be emitted from AST shape alone.

Tests:
  - ailang-core: +8 spec_drift tests; existing 12 unchanged
  - ail unit (3): rewritten in lockstep — assert (own T)/(effects IO)
    landmarks, FORM-A SPECIFICATION header, FORM_A_SPEC body verbatim
  - ail e2e merge_prose_prints_framed_prompt: rewritten — assert
    `(module foo` + `FORM-A SPECIFICATION` instead of `ailang/v0`
  - Workspace: all green

Richer integration paths (LLM tool-use, MCP server, LSP) were named
in the design discussion and deferred per "kiss". All three layer
additively on the static-prompt path; static prompt remains the
lowest-common-denominator fallback.

docs/DESIGN.md

@@ -512,6 +512,34 @@ projection) and `ail merge-prose <m.ail.json> <edited.prose.txt>`
 (the mediator-prompt composer); both are listed in the CLI section
 below.
 
+**Iter 20f update — Form-A spec embedding.** The 20d `merge-prose`
+prompt instructed the LLM to emit JSON-AST and offered a 12-line
+schema-essentials reminder; that combination did not give a
+foreign LLM enough to produce valid output. 20f revises this:
+
+- The LLM emits **Form-A** (the canonical authoring surface), not
+  JSON. JSON-AST stays the only hashable artefact, but it is not
+  a writing surface. The user runs `ail parse foo.new.ailx`
+  before `ail check` to produce the canonical JSON.
+- `crates/ailang-core/specs/form_a.md` is the complete LLM-targeted
+  Form-A specification — grammar, every term / pattern / type /
+  def keyword, schema invariants, pitfall catalogue, four
+  few-shot modules drawn from `examples/*.ailx`. It is exported
+  as `ailang_core::FORM_A_SPEC` and embedded verbatim in every
+  `merge-prose` prompt.
+- `crates/ailang-core/tests/spec_drift.rs` walks every variant of
+  `Term`, `Pattern`, `Type`, `Def`, `Literal` via exhaustive
+  `match` and asserts an anchor for each appears in the spec.
+  The exhaustive match is the load-bearing piece: adding a new
+  variant without updating the match fails compilation in this
+  test, before its assertions even run. Hand-written content,
+  mechanical drift detection.
+
+The discussion of richer integration paths (LLM tool-use, MCP
+server, LSP) was deferred — all three layer additively on the
+static-prompt path 20f ships, which remains the
+lowest-common-denominator fallback that always works.
+
 ## Decision 7: redundancy removal — `Term::If` is not a primitive
 
 **Status: REVERTED in Iter 14g.** This decision was made on shaky grounds —