Implement goto statement and labeled statements

[Copilot]

goto was parsed by the grammar but had no action code — no AST nodes were created. Labeled statements (label:) were similarly unimplemented.

Changes

• AST nodes: GotoStatement emits Jump to a resolved label; LabeledStatement defines a label and wraps the subsequent statement
• Grammar actions in CParser.jay for GOTO IDENTIFIER ';' and IDENTIFIER ':' statement
• Label resolution: Function-scoped Dictionary<string, Label> in FunctionContext. Both goto and label definitions resolve through ResolveGotoLabel/DefineGotoLabel on EmitContext, so forward references work naturally via shared Label objects.
• Validation: CheckLabels() after function emission reports undefined labels (error 9999) and duplicate labels (error 140)
• AGENTS.md: Removed "No goto" from Known Limitations

Example

void main() {
    int x = 0;
    goto skip;
    x = 42;        // skipped
skip:
    // x == 0

    int i = 0;
loop:
    i = i + 1;
    if (i < 5)
        goto loop;
    // i == 5
}

Tests

9 cases: forward goto, backward goto, multi-label chains, nested block jumps, loop breakout, and error cases for undefined/duplicate labels.