feat: add where clauses for generic function bounds

[kacy]

Adds the where clause spelling for generic function bounds, closing the last item of the traits plan.

what it does

fn f[T](v: T) -> String where T: Sized + Display: now parses and means exactly what the inline spelling fn f[T: Sized + Display](v: T) means. The clause is desugared at parse time onto the same generic_param AST nodes the bracket spelling produces, so the checker and emitter need zero changes — bound enforcement flows through the existing E226/E222 machinery. Multiple clauses separated by commas, and +-joined bounds within a clause, both work; inline and where-clause bounds on the same parameter merge.

where is a contextual keyword (matched as an identifier after the signature), so existing code using where as a name is unaffected. Clauses attach to functions and methods (impl methods route through the same declaration parser); struct, interface, and impl headers keep inline bounds only, which docs/limitations.md now states.

A clause naming something that is not a declared type parameter is a new error, E264, with a hint to declare it in the bracket list.

noticed, not fixed

impl Sized for Box[Int] (an impl on a generic instance) does not satisfy a T: Sized bound at a call site instantiating T = Box[Int] — E226 fires. Verified pre-existing: the inline spelling fails identically. The example uses a plain struct instead.

what was tested

• tests/cases/test_where_clauses.pith golden: single clause, multi-bound clause (+), multiple comma-separated clauses, and a where-clause method on an impl, all executed through interface dispatch.
• tests/invalid/where_unknown_param.pith — E264 on a clause naming an undeclared parameter.
• tests/invalid/where_bound_violated.pith — E226 on a call violating a where-declared bound, proving the desugar reaches existing enforcement.
• Formatter round-trips the where spelling (fmt on all changed files, no diff churn).
• make bootstrap-verify — fixed point holds after make refresh-bootstrap-seed (the parser change re-seeds).
• make check-invalid-only (53 passed) and make run-regressions-only (370 passed).
• examples/generics.pith gained a where-clause demo; examples/expected/generics.txt regenerated.
• docs: docs/limitations.md interface-depth entry rewritten for the two spellings; docs/errors.md gained the E264 section.