Support strict type contracts on named expressions

[Seddryck]

Add ! as an explicit strict modifier to named expression signature contracts.

%define :text! :> short-name(length -> :integer! = 3) -> :text! =
    upper | first-chars(@length);

Acceptance criteria:

• Support strict pipeline input, explicit parameter, and output contracts independently and in combination with ordinary coercing contracts.
• A strict contract checks compatibility without conversion, using is-type semantics and canonical type-family relationships rather than exact underlying runtime-type equality.
• A mismatch raises a contract error identifying the named expression, boundary/parameter, expected type, and actual type. Reject statically invalid contracts during binding where possible; runtime mismatches are evaluation errors.
• Strict :integer! accepts integer 5 and rejects text "5" and #null. An omitted parameter still uses its default, which must satisfy the contract.
• Strict :numeric! accepts values belonging to the numeric family.
• Errors stop evaluation; they are not #null/#false results and are not guard-style input preservation.
• Keep :type descriptors and existing coerce(:type)/is-type(:type) behavior unchanged. ! modifies the signature contract; it is not a new global behavior for type descriptors.
• Cover mixed strict/coercing boundaries, type families, null, defaults, failure diagnostics, and scope restoration.
• Document strictness separately from nullability and optionality; nullable contract syntax remains deferred.

Stage 7 of 8. Depends on #999.