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/contracts/0013-typeclasses.md

@@ -103,21 +103,26 @@ Three invariants make this work:
    bare `Var "show"` post-mono that codegen later rejects with
    `unknown variable`.
 
-2. **Post-mono synthesised body cross-module references may bypass
-   the source template's `import_map`.** `prelude` does not import
-   user modules (the auto-injection runs one-way: user workspaces
-   import prelude, never the inverse). But a synthesised body for
+2. **Post-mono synthesised body cross-module references are resolved
+   once, in `lower_to_mir`.** `prelude` does not import user modules
+   (the auto-injection runs one-way: user workspaces import prelude,
+   never the inverse). But a synthesised body for
    `prelude.print__<UserType>` references `<user_module>.show__<UserType>`,
-   created by mono — not by the prelude source. Codegen's
-   cross-module name-resolution must accept this: at
-   `crates/ailang-codegen/src/lib.rs::resolve_top_level_fn` (Var-
-   resolution), `::lower_app`'s cross-module call arm, and
-   `::synth_with_extras`'s Var arm, the resolution first tries the
-   current module's `import_map`, then falls back to a direct
-   `module_user_fns` / `module_def_ail_types` lookup against the
-   prefix. Both ends were independently typechecked under their own
-   module contexts before mono ran; the cross-module reference is a
-   post-mono construct, not a source-language one.
+   created by mono — not by the prelude source — so the call crosses a
+   module boundary the source template did not. This reference is
+   resolved at lowering time:
+   `crates/ailang-check/src/lower_to_mir.rs::classify_callee` resolves
+   the callee against the workspace and emits
+   `Callee::Static { module, fn_name, .. }` carrying the resolved owner.
+   Codegen consumes that identity off the MIR `App` node and does
+   **not** re-resolve it — the former codegen-side
+   `import_map`-then-`module_user_fns` fallback (at `resolve_top_level_fn`,
+   `lower_app`'s cross-module arm, and the now-deleted
+   `synth_with_extras`) is retracted, per the
+   [check→codegen boundary](0018-check-codegen-boundary.md). Both ends
+   were independently typechecked under their own module contexts before
+   mono ran; the cross-module reference is a post-mono construct, not a
+   source-language one.
 
 3. **FreeFnCall synth pushes one residual per declared forall-
    constraint.** At `crates/ailang-check/src/lib.rs`'s synth Var arm