feat: wildcard type (?) and type parameter (<?>) support

[SchoolyB]

Summary

Grayscale supports wildcard types ? (STANDARD.md §7.9) and type parameters <?> (§7.10) for generic-style functions. The LSP has no awareness of either feature.

Wildcard Types

do identity(x ?) -> ? {
    return x
}

do first(arr [?]) -> ? {
    return arr[0]
}

? is valid only in function parameter types and return types. All ? in a signature bind to the same concrete type.

Type Parameters

do make(T <?>) -> ^? {
    return new(T)
}

mut p = make(Point)  // returns ^Point

<?> allows passing a struct type name as an argument. Cannot mix with value parameters.

Scope

• Recognize ? as a valid type in function signatures for syntax awareness
• Recognize <?> parameter syntax
• Add hover docs explaining both features

[SchoolyB]

The LSP half of this shipped earlier. The tree-sitter grammar also needed a change, now in grayscale-lang/tree-sitter-gray@0090150.

wildcard_type: '?' already existed, but there was no rule for the <?> type-parameter annotation, so do make(T <?>) produced 3 ERROR nodes. Added a type_parameter rule, accepted in parameter position only.

The pinned grammar rev in zed-extension/extension.toml is updated, so Zed picks this up.

Verified against the 603 valid programs in the Grayscale integration suite: parse failures drop from 134 to 115 with zero regressions. The repository previously had no tests; test/corpus/features.txt now covers all seven features.