mecatui: shrink ui.Model so the reducer stops copying 66 KB per message #2411

Problem

ui.Model is 66,400 bytes and is passed by value through the Bubble Tea reducer chain (Update → update → afterEvent → dispatchNonInputMsg → onStreamMsg → updateStreamEvent → …). Each value-receiver frame copies it, so every message, keystroke, and stream delta copies 66 KB several times.

• A keystroke frame benchmark spends ~0.6 ms and ~1.8 MB of allocation in the reducer alone.
• The event goroutine's stack doubles to 4–8 MiB; most of it sits freed in the stack pool.

Proposal

Move the cold sub-states behind pointers (schedule, worktrees, overlay states, possibly prompt) so the Model falls under ~20 KB. This needs care wherever a reducer relies on value-copy semantics (copy-on-write or explicit clones there).

Acceptance

• unsafe.Sizeof(ui.Model{}) stays under a guard test threshold.
• The keystroke frame benchmark shows fewer bytes per op.
• No behavior change (existing suites and goldens).