feat(visualize): add a D2 emitter behind the graph emitter seam

[joshua-temple]

Problem

cascade graph renders only Mermaid. A high-fidelity, branded rendering path (the crucible teal-dark style) needs D2 source as its boundary, emitted by the main binary so a sidecar renderer can turn it into SVG or PNG.

Fix

Add a D2Emitter behind the existing visualize.Emitter seam and select it with cascade graph --format d2. It is stdlib-only string generation (no D2 dependency in the main binary), mirroring the Mermaid emitter:

• Crucible shape vocabulary per node kind: 3d state slabs (validate/build/stage), hexagon deploy invokes, rounded dashed environment plates, red hotfix slabs, small entry and double terminal markers, opaque external-repo nodes.
• Distinct edge classes for flow, promotion, optional, external, notify, and hotfix divergence/rejoin.
• Cross-repo groups render as D2 containers with container-qualified edge endpoints.
• Deterministic output (ranges ordered slices, never a map). The branded palette is fixed; the Mermaid path stays the theme-driven, GitHub-native renderer.

Verification

• Golden-output unit tests across all four granularities and topologies (jobs, stages, env with divergence, cross-repo), plus determinism, structural-validity, and error-path tests.
• graph --format d2 command tests, including the JSON envelope and cross-repo lanes.
• New e2e scenario 58-graph-d2-render exercises cascade graph --format d2 end to end (deterministic source, read-only).
• go build ./..., go test ./..., and golangci-lint run ./... clean on both the root and e2e modules.

Draft: opened for maintainer review. Sidecar binary, ephemeral install, and release publishing are intentionally out of scope here.