[contursofran]

Fixes #696

What

• internal/app/diagram/seq.go: ParseSeq (A -> B: text, A --> B: text reply, A -> A: text to itself) and SeqView (actors, dashed lifelines, arrows, dashed replies, self loop), from the spike, Painter only, tokens only, chat text size. Limits: 6 actors, 16 messages, actor 28 and message 40 characters, wider than the column → code block.
• Chat.seqBlock in chatview, the same shape as flowBlock.
• Skill kit/skills/chat-writing: a Diagrams section for contr-flow (syntax from diagram.Parse in main) and contr-seq, with limits and one example each.
• D287, ChatView.md Diagrams section. Sheet part ChatSeq, see.sh fixture chat-seq (the flow fixture now shares a helper).

Tests

• TestParseSeq, TestParseSeqErrors, TestSeqLimits, TestSeqView, TestSeqBlock.
• e2e: scripts/see.sh chat-seq dark|light: pass, all labels checked in the PNGs.

Pre-check

1. Tests: saw fail with the change undone: TestSeqBlock (seqBlock not wired), TestParseSeq (reply flag), TestSeqLimits (actor limit).
2. Plan: both steps done; limits stated in the spec.
3. Contract: contr-seq, label "Sequence diagram", D287.
4. Up to date: main merged; no open PR touches internal/app/diagram.
5. Diff read: no leftovers.
6. Coverage: issue step 2 done.
7. Duties: nothing removed; the skill's line "diagram blocks show as code" now names the two blocks that draw.
8. Evidence: sheets and see PNGs taken after the last code change.