feat(check): mir.1a — typed-MIR types + post-mono lower_to_mir + elaborate_workspace (additive)

First half of spec iteration mir.1 (docs/specs/0060-typed-mir.md,
plan docs/plans/0114-mir.1a-mir-infrastructure.md). Purely additive: a
new leaf crate plus a post-mono lowering walk plus the front-end entry.
Nothing consumes MIR yet — codegen and the CLI are untouched — so the
whole existing suite stays green (102 test-result-ok lines, 0 failed;
the one ignored is the #49 pin, which lifts at mir.4). mir.1b flips
codegen to consume MIR and deletes the synth_with_extras mirror.

What lands:
- `ailang-mir` (new leaf crate, depends only on ailang-core): MTerm /
  MArg / MArm / MLoopBinder / MNewArg / Callee / Mode / StrRep / MirDef
  / MirModule / MirWorkspace, structurally complete over all 17 Term
  variants plus the dedicated Str split (MTerm::Str{lit,rep}). Every
  later-iteration annotation field exists now at a mir.1 default
  (App.callee = Indirect, MArg.mode/consume_count = Owned/1, Str.rep =
  Static, New.elem = None); the struct shape will not change across
  mir.2–mir.5, only the values. MTerm::ty() reads the carried type.
- `ailang-check::lower_to_mir`: the post-mono walk. Carries no second
  type engine — it calls the canonical `synth` per node with fresh
  throwaway inference scaffolding (subst/counter/residuals/…), applies
  the substitution, and threads only the lexical locals/loop_stack,
  maintained by the same push/pop rules synth uses (Let, Loop, Lam,
  LetRec, and Match via synth's own type_check_pattern). This is the
  "one engine, not two" property the milestone buys, on the producer
  side.
- `ailang-check::elaborate_workspace`: the single front-end entry —
  check (diagnostics gate) → desugar+lift → monomorphise → lower_to_mir
  — transcribed from the CLI build path. check_workspace stays for the
  diagnostics-only callers.
- Three ty-fill pins (crates/ailang-check/tests/lower_to_mir_ty.rs)
  load the #51/#53/#49 witnesses as fixtures and elaborate the whole
  workspace, so lower_to_mir is exercised over the full prelude+kernel,
  not just the tiny witness. Two new witness fixtures
  (examples/new_{rawbuf_size_only,counter_user_adt}.ail); #49 reuses the
  existing loop_recur_str_binder_no_leak_pin.ail.

Deviations from the plan, all forced by ground truth and verified
against the live code before commit:
- FnDef.export is Option<String>, not bool — dropped from MirDef (the
  plan's note allowed this fallback). Codegen reads FnDef.export
  directly, unaffected.
- fn_param_types did not exist — lifted the param-type extraction into
  a shared pub(crate) fn and rewired both mono.rs sites
  (collect_mono_targets, collect_rewrite_targets) to it, so the
  param-type source cannot drift between mono and lower_to_mir. The
  helper unwraps a top-level Forall then reads Type::Fn.params —
  identical to the inner_ty extraction it replaces; the early-return
  non-fn guard is preserved at both sites. Behaviour-preserving (whole
  suite green; this is the only edit to existing mono behaviour).
- lower_module mirrors two more mono.rs scaffolding rules the plan
  under-transcribed: skip intrinsic-bodied fns (they are
  signature-only; lowering them would hit synth's Term::Intrinsic
  guard) and seed env.current_module + env.globals + env.imports per
  module so synth's Var lookup resolves post-mono specialisations
  (e.g. compare__Int) in the prelude.
- The #53 pin asserts the lowered (new Counter 42) init carries ty
  Counter, not that it is an MTerm::New: a monomorphic `new` over a
  user ADT is desugared to a dotted call `(app Counter.new 42)` before
  lower_to_mir runs (desugar.rs:1078-1098), so it lowers to MTerm::App.
  MTerm::New survives only for a polymorphic `new` carrying a written
  NewArg::Type (the #51 RawBuf path, covered by the rawbuf pin). The
  pin's intent — ty-fill correctness — is preserved; the structural
  variant is mir.2/mir.5 territory.
- ailang-mir uses workspace-inherited Cargo fields for consistency with
  the existing crates (added it to [workspace.dependencies]).

refs #49

Cargo.lock

@@ -23,6 +23,7 @@ name = "ailang-check"
 version = "0.0.1"
 dependencies = [
  "ailang-core",
+ "ailang-mir",
  "ailang-surface",
  "indexmap",
  "serde",
@@ -58,6 +59,13 @@ dependencies = [
 name = "ailang-kernel"
 version = "0.0.1"
 
+[[package]]
+name = "ailang-mir"
+version = "0.0.1"
+dependencies = [
+ "ailang-core",
+]
+
 [[package]]
 name = "ailang-prose"
 version = "0.0.1"