feat(render): add self-contained interactive HTML viewer

[JAORMX]

Summary

Add modelith render --format html as an offline, self-contained interactive viewer while keeping Markdown the default.

• Embed HTML/CSS/vanilla JS in the existing static binary; no new dependencies, server, or external assets.
• Show directed relationships with declared cardinalities and ownership, subtype edges, self-relationships, and external entities.
• Add entity details, search, pan/zoom, draggable nodes, deterministic grid/flow arrangements, themes, and relationship highlighting.
• Present model-wide invariants, enums, glossary, scenarios, and imports in counted accessible tabs.
• Integrate collapse into the details panel: a persistent header control, a narrow desktop reopen rail, and a compact mobile expand row. Collapsing preserves graph state.
• Support existing --stdout, --out, and --check workflows and format completion; update CLI docs and ADR-0020.

Model content is encoded with Go's default HTML-safe JSON escaping and rendered through DOM text APIs. Schema and model structs are unchanged. The HTML golden lives in package testdata rather than examples to avoid adding a large generated output to the example workflow; Go tests check it byte-for-byte.

Validation

• node --test internal/render/html/viewer_test.cjs: 30 passing tests.
• task check: vet, staticcheck, golangci-lint, race-enabled Go tests, strict example lint, and Markdown render checks pass. The final validate-plugin step is blocked because claude is not installed; no gate was bypassed.
• HTML golden regeneration/review and git diff --check pass.
• Installed locally and exercised against the mecatl domain model; rendered output was reviewed interactively by the author through screenshots and feedback.

Review and limitations

Independent agent review/fix rounds covered correctness, standards, geometry, interaction state, and UX. These were approximations, not /code-review. The final UX pass found no remaining blocker after fixing the collapsed rail's clipped keyboard-focus indicator.

The Node harness mocks DOM/geometry; real-browser layout and accessibility smoke testing remains outstanding. The initial independent security-agent review did not complete; lead inspection and escaping/hostile-content regression tests cover the encoding path, but are not represented as an independent security sign-off.

[JAORMX]

Review record (autonomous approximation, not /code-review): initial spec/standards/UX/adversarial rounds fixed HTML check recovery, directed/parallel relationships, self-relationship display, and external incoming details. Subsequent geometry and interaction rounds addressed label placement, draggable layouts, highlight state, and model-wide tabs. For the final panel-integrated collapse change, UX review found the collapsed rail clipped its keyboard-focus outline; an inset outline and width correction fixed it, with structural/contrast tests. Final UX re-review: no remaining blocker. Current validation: 30 Node harness tests pass; task check passes Go lint/vet/race tests, model lint, and render checks, but validate-plugin is blocked by missing claude. Browser-native layout/accessibility smoke and completed independent security review remain gaps, documented in the PR description.