lowering: capture check by free variables, and say when it rejects

[simontreanor]

Follow-up to #39, the option you picked: free variables plus a note.

The capture precondition compared every name mentioned inside a nested function against the names the frame binds. Two safe shapes were rejected by that; one is now fixed and the other explains itself.

Fixed — shadowing is not capture. List.map (fun n -> n + 1) xs inside a function whose parameter is also n used to block the loop, though the lambda binds its own n and shares nothing. The check now collects true free variables: subtract the nested function's parameters, locals and match captures, and add back anything it declares nonlocal or global, since those name the enclosing cell by definition and are captures even though the body assigns them.

Still rejected — capture that cannot escape. List.fold (fun acc x -> acc + x * n) 0 xs captures n but is consumed within the iteration. Proving that needs per-callee escape analysis, which is not worth the machinery.

And the silence is gone. A rejection previously left the function recursing with nothing said, so a program could keep hitting the recursion limit with only the emitted Python to find out from:

note: `collect` calls itself in tail position but keeps its recursive form: a closure in it captures `n`
note: `shadowed` calls itself in tail position but keeps its recursive form: the body rebinds `shadowed`

has_self_tail_call runs before any precondition, so a function with no self tail call is never mentioned — which is almost all of them. There are tests for both halves of that: a rejected call notes, a successful one and an ordinary function say nothing.

Plumbing: notes ride out through lowering::lower_collecting / LoweredModule.notes → lib::compile_collecting / project::CompiledProject.notes → the CLI's report_notes, printed to stderr by compile and run. lower and compile keep their existing signatures by delegating and dropping the notes, so no caller had to change.

Tests: 4 new in tests/compile.rs (shadowing no longer blocks; a rejection notes with the reason; a successful rewrite is silent; an ordinary function is silent), on top of #39's guard tests which all still pass. Full suite, clippy and fmt clean.