feat(docs): share-URL permalinks + copy-as-Mermaid + copy-as-Go-builder

Viral primitive that turns the visualizer into a sharable artifact.

- New utils/share-url.ts:
  - encodeMachine / decodeMachine (URL-safe base64 over JSON)
  - readMachineFromHash() inspects #m=... on load
  - buildPermalink(machine) emits a self-referential URL
  - machineToMermaid generates a stateDiagram-v2 snippet
  - machineToGoBuilder synthesizes a NewMachine[struct{}] chain
- JsonImporter.vue: on mount, prefer machine in URL hash before the
  first-visit sample. Permalinks always land on the shared machine.
- MachineJson.vue: panel grows three new copy buttons (Permalink,
  Mermaid, Go) alongside Copy JSON. State tracking switched from
  boolean to a CopyKind discriminator so the right button shows
  "Copied!". 2x2 grid layout via .share-actions.
- ARIA: panel-header collapse button now exposes aria-expanded;
  decorative SVGs marked aria-hidden.
- Clipboard refactored into a single copyText helper that retains
  the navigator.clipboard → execCommand fallback path.

Source: 4-expert website review — Product dimension top-3 ROI move
("share-URL + LZ-compressed JSON in hash" + "copy as Mermaid" +
"copy as Go builder"). Compression deferred until size becomes a
concrete issue.

Author: felixgeelhaar