Add type contracts to named expression parameters

[Seddryck]

Allow explicit parameters to declare Expressif type descriptors, including parameters with defaults.

%define normalize-name = trim | upper;
%define short-name(length -> :integer = 3) =
    &normalize-name | first-chars(@length);

Acceptance criteria:

• Parse name -> :type, optionally followed by = default; retain untyped declarations.
• Reuse the canonical type descriptors and supported conversions used by coerce. :integer and :text remain descriptors, not new type names specific to definitions.
• For coercing parameter contracts, convert the supplied/default value before binding the local parameter. Failed scalar conversion yields #null, which enters the body as a normal value; it neither triggers the default nor skips the body.
• Preserve the distinction between recoverable conversion failure and structural/unsupported-contract errors. Do not introduce a universal errors-to-null rule.
• Validate unknown type descriptors and unsupported contracts during binding where possible; make supported target types explicit.
• Preserve existing coerce(:type) and is-type(:type) behavior.
• Test successful conversion, already compatible values, failed conversion, explicit null, typed defaults, unknown types, and propagation into the body.
• Document that a coercing annotation is not a non-null guarantee. Strict contracts and nullable-type syntax are outside this stage.

Stage 5 of 8. Depends on #997.