feat: named arguments and named return values awareness

[SchoolyB]

Summary

Grayscale supports named arguments at call sites (STANDARD.md §7.2.5) and named return values in function signatures (§7.3.4). The LSP has limited or no awareness of either.

Named Arguments

do connect(host string, port int = 8080, verbose bool = false) { }

connect(host: "localhost", verbose: true)  // port uses default
connect("localhost", verbose: true)        // positional + named mix

Rules:

• Positional arguments must come before named arguments
• Not supported for builtins or stdlib functions

Named Return Values

do divide(a, b int) -> (quotient int, remainder int) {
    mut quotient int = a / b
    mut remainder int = a % b
    return quotient, remainder
}

Named returns are labels only — they don't implicitly declare variables. Support grouped types.

Scope

• Recognize name: value syntax in function call arguments
• Recognize named return value syntax in function signatures
• Hover docs could show parameter names to help users write named arguments

[SchoolyB]

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

call_expression arguments were bare $._expression, so connect(host: \"localhost\") produced 3 ERROR nodes. Added a named_argument rule. Named returns already parsed via the existing named_return rule.

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.