terminal/c: fix crash on invalid enum in C API get functions

[deblasis]

What

All get functions in the C API crash when called with .invalid and a null output pointer. This affects build_info, cell, render (3 functions), row, and terminal - 7 functions total.

Also colors_get never copies partial palette entries even though the code to do so exists.

Why it crashes

The get functions use inline else on an enum switch to dispatch to a typed helper. inline else generates code for every variant, including .invalid. When .invalid is used with null as the output pointer:

@ptrCast(@alignCast(out))  // out is null

This casts null to a non-optional *void - safety panic. The typed helper would just return .invalid_value without touching the pointer, but we never get there.

Fix

Return early before the switch for .invalid, mark it unreachable in the switch so inline else doesn't generate code for it:

if (data == .invalid) return .invalid_value;

return switch (data) {
    .invalid => unreachable,
    inline else => |comptime_data| getTyped(...),
};

For colors_get: removed the structSizedFieldFits guard around the palette copy. That check requires the full 256-entry array to fit, but the inner logic already calculates how many entries fit and copies only those. The guard was preventing it from ever running for truncated structs.

Testing

All existing tests pass on Windows, macOS (arm64), Linux (x86_64). The get invalid tests that were crashing now pass.

Notes

Draft tracking PR on the fork. Will clean up before submitting upstream.

[deblasis]

Cherry-picked into Windows stack branch 003-windows/cherry-pick-c-api-fix.

[deblasis]

Closing - upstream fixed the same crash in commit 5828352.

We added an early return before the switch then marked .invalid as unreachable inside it. Mitchell just added .invalid => .invalid_value as a single switch arm. Same fix, half the code, all logic in one place.

We also missed osc.zig which had the same pattern. Upstream caught all 6 files.

Lesson: when handling a new enum variant, add it directly in the switch instead of guarding before it. And grep the whole directory when fixing a repeated pattern, not just the files you already know about.