Make QuantitySignature construction independent of caller ordering #1946

Summary

QuantitySignature is only correct if callers call check_quantity_param for every parameter in order on both the built-in and extern paths. This relies on the caller following a protocol, against the project rule that invariants are enforced by the API.

Current state

• FunctionSignature::try_new already guarantees each dimension variable is bound before use.
• The remaining internal-error site is in tir/dim_check/builtins.rs (the QuantitySignature walk shared with plugin extern signatures).

Proposed direction

Re-encode monomials over binder slots when FunctionSignature is validated, so a QuantitySignature can only be constructed through one validated walk, and the per-parameter call order no longer matters.