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/parse.rs

@@ -130,6 +130,29 @@ pub fn parse(input: &str) -> Result<Module, ParseError> {
     Ok(m)
 }
 
+/// Parse a single form-A term — the concrete-syntax counterpart of
+/// [`Term`]. This is the dual of [`crate::print::term_to_form_a`] and is
+/// 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
+/// input must consume to EOF after the term — extra trailing tokens
+/// produce [`ParseError::Unexpected`].
+///
+/// Round-trip: `parse_term(term_to_form_a(t)) ≡ t` for every term `t`
+/// the surface can express.
+pub fn parse_term(input: &str) -> Result<Term, ParseError> {
+    let toks = tokenize(input)?;
+    let mut p = Parser::new(&toks);
+    let t = p.parse_term()?;
+    if p.cur < p.toks.len() {
+        return Err(ParseError::Unexpected {
+            expected: "end of input".into(),
+            got: tok_label(&p.toks[p.cur].tok),
+            pos: p.toks[p.cur].span.start,
+        });
+    }
+    Ok(t)
+}
+
 fn tok_label(t: &Tok) -> String {
     match t {
         Tok::LParen => "`(`".into(),