feat: C interop support (import c"...", c. prefix)

[SchoolyB]

Summary

Grayscale supports C interop (STANDARD.md §8.6) — importing C headers and calling C functions. The LSP has no awareness of this feature.

Example

import c"stdio.h"
import c"stdlib.h", c"string.h"

do main() {
    c.puts("hello from C")
    c.printf("value: %d\n", 42)
    mut home string = c_string(c.getenv("HOME"))
}

Key Details

• import c"header.h" — system header (angle-bracket include)
• import c"./mylib.h" — local header (quoted include)
• All C functions accessed via c. prefix
• Module name c is reserved for C interop

Scope

• Recognize import c"..." syntax (no false diagnostics)
• Recognize c. prefix as valid module access
• Add hover docs explaining C interop basics