feat(review-tutor): add the Structure graph

[ElbertePlinio]

Refs #62 — PR 3 of 4 (deterministic Structure graph). Stacked on #71; part of the batched Review Tutor chain.

What changed

• A List | Graph switch inside the Structure view (List stays the default at every width; the choice is remembered per tab).
• One inline SVG: changed files as nodes in a deterministic layered layout (longest-path layering, stable in-layer order by directory then path; byte-identical coordinates for identical snapshots), connections as straight lines with a small arrowhead, colored by status like the list (type-only at reduced opacity). Multi-layer edges route below intermediate rows and back edges below the drawing, so no line ever crosses a label. No physics, no motion.
• Selecting a node or an edge (click, Enter, Space; Escape clears) shows the same evidence rows and Open in Diff the list uses — same jump path and landing marker. Exactly one selection, one ember.
• Honest size bound: more than 60 drawable nodes or 200 connections shows one sentence instead of a hairball; zero drawable nodes shows one sentence.
• Elements added: exactly four (switch, SVG, reused evidence panel, size-bound note). No legend, clusters, filters, tooltips, zoom, or minimap — per branding-visual/DESIGN-PRINCIPLES.md.

Tested

• Layout determinism, layering on a DAG and a cycle, in-layer ordering, edge routing never intersecting a node rect, size bounds, zero-drawable copy, selection exclusivity, Escape/keyboard order, aria labels, mode persistence, evidence reuse + Open in Diff from the graph, no innerHTML; list mode tests unchanged. Rendered proof at 1440 / 860 / 390 / 360@4x for loaded, node-selected, edge-selected, size-bound, and keyboard-focus states.
• bun run check: 490 passed | 25 skipped, 81.51% statements.

Not tested

• Graphs near the size bound on real repositories (fixture-driven renders only).

Local review: serious user-facing class — three independent reviewers (correctness, tests/conformance/doctrine, UI/accessibility with rendered inspection). Human gate: merge only on Elberte's authorization.

[ElbertePlinio]

Ready for human review at c4a2f82. Panel + verification clean; low-severity follow-ups listed on #62.