feat: tagged enum support (variants with data payloads)

[SchoolyB]

Summary

Grayscale supports tagged enums (STANDARD.md §3.2.4) — enum variants that carry associated data payloads. The LSP has no awareness of this feature.

Example

const Shape enum {
    Circle(float)
    Rect(float, float)
    Point
}

mut s Shape = Shape.Circle(3.14)

when shape {
    is Shape.Circle(radius) {
        println("Circle with radius ${radius}")
    }
    is Shape.Rect(w, h) {
        println("Rectangle ${w} x ${h}")
    }
    is Shape.Point {
        println("Just a point")
    }
}

Scope

• Update hover docs for enum to mention tagged variants
• Symbol scanner should recognize variant payloads in enum declarations
• Awareness of destructuring patterns in when/is blocks

[SchoolyB]

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

enum_value was identifier [= expr] with no payload form, so Circle(float) failed to parse. Added a variant_payload rule; payloads and explicit values are mutually exclusive, matching the spec.

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.