Revise lib-igs-author-plan.md per rich-menu-editor-plan §5

[ScottMorris]

Part of the rich menu editor epic. Plan: docs/rich-menu-editor-plan.md §5.

Revise docs/lib-igs-author-plan.md to match the verified state of libhdmv and the rich-menu-editor design decisions:

• Phase A no longer claims igs::write_object_segment / write_palette_segment exist (the igs crate is decode-only); add the segment-writer work item (Phase A0) and re-estimate
• Phase D integrates from MenuDocument + pre-rendered per-state RGBA bitmaps (via the MenuCompiler seam), not legacy Menu/MenuButton
• AuthorButton carries Vec<ButtonImage> frame sequences per state (len 1 = static)
• AuthorPalette::Auto specifies joint quantisation across all objects in a display set
• Cross-link the libhdmv epic