fix: terminal Color type + wasm pointer size

[zubyul]

Problem

Color was refactored from u24 to packed struct(u32) with tag+payload, but terminal tests still pass bare hex integers (0xFF0000), causing 15 compile errors.

terminal_frame_ptr() returns u32 but @intFromPtr produces usize on 64-bit, causing 1 more error.

Fix

• Replace bare hex color literals with Color.RED, Color.BLACK, Color.WHITE, etc.
• Use Color{ .tag = .srgb, .payload = 0xABCDEF } for non-standard colors
• Change terminal_frame_ptr() return type to usize

Result

zig test src/terminal_wasm.zig
All 41 tests passed.

Build summary improves from 796/797 → 797/797 tests passing (for terminal module).