Iter 18c.2: linearity check + suggested_rewrites

Adds a per-fn linearity check that runs on every fn whose
param_modes are all explicit (Borrow or Own, no Implicit), tracks
each binder's consume/borrow state through the AST, and emits
two diagnostics with form-A `suggested_rewrites`:

- `use-after-consume` — a binder is referenced after it has
  already been consumed.
- `consume-while-borrowed` — a binder is consumed while a borrow
  of it is still live (sibling arg slot, or an enclosing-fn
  Borrow param).

Fns with any Implicit param skip the check entirely; that's the
back-compat lane that keeps every existing fixture green. Pure
diagnostic addition: no IR change, no codegen change, no runtime
change.

Each diagnostic carries a non-empty `suggested_rewrites` whose
`replacement` is form-A AILang text (typically `(clone <name>)`).
Adds two new public entrypoints to ailang-surface — `parse_term`
and `term_to_form_a` — so the round-trip
`parse_term(term_to_form_a(t)) ≡ t` is the contract every
emitted replacement must satisfy. Tests assert the contract.

Linearity pass runs only on modules that typechecked clean (any
upstream typecheck error suppresses it for that module — running
on partly-defined IR would produce noise).

Tests: 3 unit tests in `linearity::tests`, 3 integration tests
in `ailang-check/tests/workspace.rs`, 1 serialisation test in
`diagnostic.rs`. `cargo test --workspace` green.

Cargo.lock

@@ -20,6 +20,7 @@ name = "ailang-check"
 version = "0.0.1"
 dependencies = [
  "ailang-core",
+ "ailang-surface",
  "indexmap",
  "serde",
  "serde_json",