lowering: block-local functions carry their arity, so a partial application curries

[simontreanor]

What

A function bound by a block-local let with parameters, then applied to fewer (or more) arguments than it takes, now curries exactly like a top-level function:

let addAll xs ys =
  let pair a b = a + b
  List.map (pair 10) ys

emits _pf_map(functools.partial(pair, 10), ys) and prints [11, 12]. Before, the same program passed pyfun check and died at runtime with TypeError: pair() missing 1 required positional argument, because lowering took arity only from the module-level table and a local name answered "unknown", which is emitted n-ary as written.

How

A block-scoped local_arities table lives beside the fold pass's local_fn_defs registry and follows the same scope discipline, so an entry can only mean "this name resolves to that local function here":

• registered for let f a b = … and let f = fun a b -> … (parameter count of the binding or the lambda);
• snapshotted and restored per block (save_local_scope/restore_local_scope), and around a computation expression body, which is its own Python function;
• displaced by parameters and match-arm binders for the binder's extent (shadow_local_fns/unshadow_local_fns, now carrying both registries);
• evicted when a non-function let or a CE binder rebinds the name.

One ordering detail: a function let registers its arity before its value is lowered, so a recursive local function can partially apply itself; a non-function let evicts after its value is lowered, since let pair = pair 1 reads the previous binding.

lower_application consults local_arities first. Parameters have no entry and stay n-ary, so nothing changes for a callee whose arity really is unknown. Over-application of a local function (k 1 2 x with a 2-ary k returning a function) now emits k(1, 2)(x) instead of a three-argument call.

Tests

tests/compile.rs: the issue's program (string-level and end-to-end), the lambda spelling, over-application, a parameter shadowing the local function, a match-arm binder shadowing it, a recursive local function partially applying itself, and a non-function rebinding evicting the arity.

Docs: the src/lowering/mod.rs header comment and a new "Arity for the currying lowering" section in INTERNALS.md.

Closes #84