Extract ConversationState and renderConversation from AppLayout (step 5b)

[shellicar]

What

Extracts the conversation block state and rendering out of AppLayout into two focused, independently testable files.

ConversationState.ts — pure state

• #sealedBlocks, #flushedCount, #activeBlock move here
• transitionBlock returns a result struct ({noop, from, sealed}) so AppLayout can log without re-reading private state
• appendToLastSealed returns 'active', a sealed index, or 'miss' so the caller can log the exact target
• addBlocks, appendToActive, completeActive, advanceFlushedCount cover the remaining mutation surface

renderConversation.ts — pure rendering

• All block rendering helpers move here: renderBlockContent, buildDivider, BLOCK_EMOJI, BLOCK_PLAIN
• renderConversation(state, cols) — sealed + active blocks for the alt-buffer viewport
• renderBlocksToString(blocks, startIndex, cols) — slice-to-string for the flush-to-scroll path; uses full array for continuation checks so headers suppress correctly even for already-flushed blocks
• buildDivider(null, cols) replaces the inline DIM + FILL.repeat(cols) + RESET in render()

AppLayout after

• Holds #conversationState = new ConversationState()
• All block mutations delegate to it
• render() calls renderConversation(this.#conversationState, cols) then appends editor lines
• #flushToScroll() calls renderBlocksToString(...) and advanceFlushedCount()
• FILL constant removed; BLOCK_EMOJI/PLAIN removed; renderBlockContent/buildDivider removed

Tests

41 new tests (231 total, up from 190):

• ConversationState.spec.ts (27): initial state, addBlocks, transitionBlock (noop/seal/discard/from), appendToActive, completeActive, appendToLastSealed (active/sealed/miss/most-recent), advanceFlushedCount
• renderConversation.spec.ts (14): empty state, single block structure, continuation suppression, active block rendering, buildDivider variants

Risk

Medium. The flush-to-scroll boundary (#flushedCount) is the dangerous part — blocks flushed to scroll are permanently written and cannot be un-written. This is addressed by keeping the flush logic in AppLayout and only delegating state reads/writes to ConversationState. The continuation-check references the full sealed block array (not just the unflushed slice) so headers suppress correctly for already-flushed blocks.

[bananabot9000]

Clean extraction. Return-value APIs (TransitionResult, appendToLastSealed discriminant) are the highlight -- opaque state with expressive caller interfaces. renderBlocksToString taking the full array is the subtle-but-correct call for flush boundary continuations. One suggestion: add a test for renderBlocksToString covering continuation suppression across a flush boundary, since the PR itself identifies this as the highest-risk area. 41 tests, 706 total. Approve. 🍌