docs(design): ratify the check→codegen boundary (mir.5, typed-MIR close)

mir.5 is the typed-MIR milestone's closing iteration. Its CODE half had
already converged before the iteration began, so mir.5 ships no code —
it ratifies into the design/ ledger the boundary the code already holds.

Verified at iteration entry (empirically, not from the spec sketch):
  - all four named re-derivers grep-clean in codegen: synth_with_extras,
    synth_arg_type, type_home_module, the second infer_module_with_cross;
  - lower_workspace takes &MirWorkspace (codegen consumes MIR);
  - MTerm::New is unreachable!() — raw-buf.4 desugars Term::New to
    (app T.new …) before codegen, so there is no element-type
    re-derivation left to relocate;
  - #51 / #53 (the element-type / new-T codegen crashes) are closed;
    their residue was fixed by ee4107c / 420f75f plus the New-desugar,
    not by a separate raw-buf patch track.

Ledger work (the mir.5 deliverable):
  - NEW design/contracts/0018-check-codegen-boundary.md: the invariant
    "codegen re-derives nothing; MIR is total over what check proved;
    a codegen arm that recomputes a fact instead of reading MIR is
    drift." Ratifier: lower_to_mir_ty.rs::callee_classification_builtin_and_static.
  - 0013-typeclasses invariant 2 retracted: codegen no longer re-resolves
    cross-module names via an import_map fallback; lower_to_mir::classify_callee
    resolves the reference once into Callee::Static and codegen consumes it.
  - 0003-pipeline.md: the "lower to MIR" line names the real stage
    (elaborate_workspace → MirWorkspace → lower_workspace) and a new
    paragraph states the boundary, cross-referencing 0018.
  - INDEX.md: boundary contract row added; qualified-xref re-pointed at
    lower_to_mir + 0018.
  - codegen_import_map_fallback_pin.rs doc-comment made honest — it pins
    the post-mono AST precondition classify_callee relies on, not a
    codegen-side resolution that no longer exists. Assertions unchanged;
    the test stays green.
  - spec 0060 gains a mir.5 refinement note recording the early code
    convergence.

Acceptance criteria 1-7 of docs/specs/0060-typed-mir.md are all met.
Full workspace suite green (exit 0, 0 failed, 2 ignored); 708 passed
carried from mir.4 (no test added or removed).

Not done here (deliberately): the milestone #7 (raw-buf) subsumption
note is an external Gitea tracker write; the /boss auto-mode classifier
declined it as an unauthorised external write and it is surfaced to the
user rather than worked around. The end-to-end milestone fieldtest
remains the deliberate manual close-gate before the tracker milestone
is marked done.

refs #51 #53

design/INDEX.md

@@ -87,7 +87,7 @@ is the default.
 | data-model           | LLM author / stable           | crates/ailang-core/tests/design_schema_drift.rs | design/contracts/0002-data-model.md |
 | mangling             | codegen / stable              | crates/ail/tests/eq_ord_e2e.rs            | crates/ailang-codegen/src/lib.rs //! |
 | env-construction     | codegen / stable              | crates/ailang-check/tests/duplicate_ctor_pin.rs | crates/ailang-codegen/src/lib.rs //! |
-| qualified-xref       | codegen / stable              | crates/ail/tests/codegen_import_map_fallback_pin.rs | crates/ail/src/main.rs //! |
+| qualified-xref       | check→codegen / stable        | crates/ail/tests/codegen_import_map_fallback_pin.rs | crates/ailang-check/src/lower_to_mir.rs (classify_callee) + design/contracts/0018-check-codegen-boundary.md |
 | frozen-value-layout  | embedding ABI / one-way-frozen| crates/ailang-codegen/tests/embed_record_layout_pin.rs | design/contracts/0006-frozen-value-layout.md + runtime/rc.c §layout |
 | float-semantics      | LLM author / stable           | crates/ail/tests/eq_float_noinstance.rs   | design/contracts/0005-float-semantics.md |
 | typeclasses          | LLM author / stable           | crates/ail/tests/show_no_instance_e2e.rs  | design/contracts/0013-typeclasses.md |
@@ -99,6 +99,7 @@ is the default.
 | embedding-abi        | embedding host / stable       | crates/ailang-codegen/tests/embed_record_layout_pin.rs | design/contracts/0003-embedding-abi.md |
 | scope-boundaries     | architect+author / stable     | crates/ailang-core/tests/effect_doc_honesty_pin.rs | design/contracts/0010-scope-boundaries.md |
 | verification         | architect / stable            | bench/architect_sweeps.sh                 | design/contracts/0014-verification.md |
+| check-codegen-boundary| architect / stable           | crates/ailang-check/tests/lower_to_mir_ty.rs (callee_classification_builtin_and_static) | design/contracts/0018-check-codegen-boundary.md |
 
 ## Models
 | id | consumer / lifetime | link |