fix: decode unpacked indexed PICT pixmaps

[benletchford]

Closes #606.

Root cause

The PICT decoder treated every BitsRect / BitsRgn opcode as a 1-bit BitMap. Version 2 pictures can set the high bit of rowBytes and supply an indexed PixMap with a color table and unpacked pixel data. Reading that structure as a BitMap desynchronized the opcode stream; a later pixel word could then be interpreted as a reserved opcode with a huge length and overflow the parser position.

Change

• Detect indexed PixMap forms of BitsRect and BitsRgn.
• Share the existing indexed color mapping, clipping, transfer, and blit paths with PackBitsRect while reading BitsRect rows uncompressed.
• Teach byte-slice PICT scanning to skip the same PixMap structure correctly.
• Stop safely when a reserved-opcode payload length would overflow.
• Add regression tests for an unpacked 4-bit PixMap and the overflow boundary.

The implementation follows Inside Macintosh: Imaging With QuickDraw (1994), Appendix A, pp. A-13 and A-17.

Qualification

The checksum-pinned, pure-68K Falcon MC promotional demo now:

• renders its authentic monitor-depth prompt;
• switches through its requested 16-color path;
• renders its promotional menu and bundled controls reference;
• enters live Instant Action cockpit gameplay;
• accepts the bundled view and Space trigger inputs while simulation continues.

Before this change, startup panicked while decoding PICT 132 (BitsRect, rowBytes=$8004, 4-bit indexed PixMap).

Validation

• cargo fmt --check
• New focused regressions: passed
• cargo test --lib: 2,950 passed, 3 ignored
• cargo check --no-default-features: passed