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.

crates/ailang-surface/src/print.rs

@@ -31,6 +31,18 @@ pub fn print(module: &Module) -> String {
     out
 }
 
+/// Print a single [`Term`] in form (A) — the dual of
+/// [`crate::parse::parse_term`]. Used by callers that produce form-A
+/// snippets out-of-band, e.g. the `suggested_rewrites` payload of
+/// `ail check --json` (Iter 18c.2). The output is round-trip stable
+/// (`parse_term(term_to_form_a(t)) ≡ t`) but does not include a trailing
+/// newline; callers that want one append it themselves.
+pub fn term_to_form_a(t: &Term) -> String {
+    let mut out = String::new();
+    write_term(&mut out, t, 0);
+    out
+}
+
 // ---- helpers --------------------------------------------------------------
 
 fn indent(out: &mut String, level: usize) {