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

docs/specs/0060-typed-mir.md

@@ -407,6 +407,21 @@ class and deletes the matching codegen re-derivation.
 > `Str` values reaching such a binder are literals (promoted here) and
 > already-heap expressions (`str_concat`, explicit `clone`).
 
+> mir.5 refinement (discovered at iteration entry, 2026-06-01): the
+> **code** half of mir.5 — "element-type / `Term::New` fully on MIR" —
+> had already converged before mir.5 began. The `New.elem` element-type
+> residue was closed by the #51/#53 bug fixes (`ee4107c` honours the
+> written element type-arg in check; `420f75f` resolves the dotted
+> `T.new` user-ADT callee) together with the raw-buf.4 `New`
+> desugar, which rewrites `Term::New` into `(app T.new …)` *before*
+> codegen — so `MTerm::New` is `unreachable!()` at codegen and there is
+> no element-type re-derivation left to relocate. At mir.5 entry all
+> four named re-derivers are grep-clean, `lower_workspace` takes
+> `&MirWorkspace`, and #51/#53 are closed. **mir.5 is therefore
+> ledger-only:** it ratifies into the `design/` ledger the boundary the
+> code already holds. No new code ships; the acceptance criteria below
+> are met by the landed mir.1–4 work plus the #51/#53 fixes.
+
 mir.5 carries the ledger work as part of the milestone (per CLAUDE.md:
 contract changes ship with the iteration that needs them):