Mermaid post-slice-4 polish: silent-misparse fixes + flowchart doc-parity gaps #32

Follow-up batch from the four-slice mermaid project (PRs #4, #17, #26, #31) — the whole-branch reviews' deferred items plus a doc-parity gap analysis against https://mermaid.ai/open-source/syntax/flowchart.html, confirmed by executing every doc example through our renderer (crates/mermaid/examples/doc_gallery.rs, committed on main).

Fix first — silent divergences ✅ DONE in #44

1. Circle/cross edge-ends parse as phantom nodes — terminators now bind to the edge (A--oB = circle-ended edge to B, matching mermaid's documented rule).
2. Edges to subgraph ids create phantom nodes — now a loud error naming the subgraph. Actual cluster-boundary edge routing remains deferred (below).

Cheap doc-parity adds ✅ DONE in #44

Subroutine [[…]]; multi-length arrows; invisible ~~~; bidirectional <--> / o--o / x--x; no-space label spellings; YAML front-matter skip (error lines preserved); classDef default auto-apply. Also fixed en route: quote-aware ; statement splitting (the docs' A["This is a #35; test"] example errored mid-quote) and -.-/=== no longer render a phantom arrowhead.

Deferred items (the remaining work; per-slice details in the worktree ledgers)

Flowchart / structural

• Actual edge-to-subgraph routing (edge attaches to the cluster box; today it errors loudly).
• Flowchart-onto-boxgraph unification — also the natural moment for the two state items below marked (†).

State: (†) ;-in-transition-label truncation; (†) header can't chain same-line statements; notes can target synthetic __start_N ids; composite/node collision guards untested.

Class: same-line class A { } errors; multiplicity labels sit on opposite sides of an edge; 14px multiplicity offset overshoots short segments; classDef default merge semantics — we apply default wholesale to unclassed nodes only, mermaid merges default's properties per-property under explicit classes (cosmetic; flowchart side, filed from the #44 review).

ER: blank-line-in-block permissiveness (confirm intent); doubled validate_id; ragged per-row key column.

Sequence (slice-3 backlog): Note over A,B,C drops the third participant; <br/> in participant displays/edge labels measured multi-line but rendered literal; self-message text reservation 4px tight; props soup lacks actor/cross/async lines.

Lib: DiagramKind::label() has zero callers (public-API removal decision).

Deliberate non-goals (do not "fix")

HTML in labels stays escaped (mermaid renders it; we never will for untrusted docs); markdown strings, FontAwesome, entity-code decoding, @{ shape: … } expanded shapes, icon/image nodes, edge ids/animations, click/linkStyle/style remain out of scope with loud errors.

Parity matrix

Post-#44 (executed 2026-07-11): 54 of 62 doc examples render, 8 error loudly, 0 silent divergences. Full per-example table in crates/mermaid/examples/doc_gallery.rs (cargo run -p ogrenotes-mermaid --example doc_gallery). Every remaining error case is in the non-goals list or the deferred list above.