IGS segment writers: palette, object, interactive composition, END (Phase A0)

[ScottMorris]

Part of the igs-author epic. The igs crate currently exposes decode only (decode.rs, types.rs); lib-igs-author-plan.md assumed write_object_segment / write_palette_segment exist, and they do not.

Add IGS segment writers, mirroring the existing decoder types:

• Palette definition segment writer
• Object definition segment writer (RLE payload via pgs::rle::encode_rle, incl. multi-fragment objects over 64 KB)
• Interactive composition segment writer (pages, BOGs, buttons, effects, UO mask)
• END segment writer
• Round-trip tests: write → existing decoder → structural equality (the decoder is the oracle)