TUI resident transcript: scroll/pin behavior is untested at the Update level and has three stuck-view gaps

[jsmestad]

Type: Bug (hardening follow-up)

Part of #2654. Five-agent review of merged #2711 (full findings on that PR); the decode/apply layer was already hardened in #2714. This issue covers the surviving scroll/pin surface.

Implemented in #3391.

What

The TUI's local transcript scroll and pin-intent reporting work on the happy path, but the behavior is untested at the Model.Update level where it actually happens, and three edge paths leave a wrong-or-stuck view (or a desynced BEAM follow state) with no signal.

Why

The pin flag drives BEAM-side auto-follow; a silent desync means "chat stopped following" bug reports with nothing in the logs. The untested seams (key routing, wire bytes, composer gating) are exactly where a refactor regresses silently. The review constructed several green-tests-wrong-behavior scenarios.

Acceptance Criteria

• Driving Model.Update with real key/wheel input against a live out channel asserts exactly one 0x5C on the scroll-away edge and exactly one 0x5D on the return edge, with no duplicates on repeated same-direction input.
  • Byte-layout tests cover EncodeGUIChatScrolledAwayFromBottom and EncodeGUIChatReturnedToBottom.
  • A table test covers the key-to-scroll mapping for j, k, Ctrl+d, Ctrl+u, G, PgUp, and PgDn, including not-handled modifier combinations.
• With InputFocused = true, navigation keys reach the composer and do not scroll the transcript, and vice versa. The 0x09 section decode has round-trip coverage including the absent-section default.
• A transcript that shrinks while the reader is scrolled up reports its automatic re-pin to the BEAM. The maxTop == 0, exact-fit, and clamp-to-bottom paths record pinReturned.
• The truncated flag is surfaced through an in-budget earlier messages hidden affordance at the top of a capped transcript.
• Same-epoch full replacement preserves the reading position by stable local message identity. Pin-intent reporting around the transition is covered, including empty-transcript and epoch behavior.
• Page-size keys scroll by the transcript content budget instead of the full body height.