feat: Zed preview panel via the visual extension API (blocked upstream on zed-industries/zed#53403)

[onokonem]

Follow-up from #30.

#30 shipped with the issue's documented fallback because zed_extension_api 0.7.0 has no WebView/panel API — visual extensions are still an open RFC: zed-industries/zed#53403 (verified against the crate source at implementation time).

Current state (fallback, working today): render/export task templates running the CLI (byte-identical to c4drill <file>), drill-through via the SVG's own xlink:href navigation, live error state from LSP diagnostics.

When the API lands

1. Add a WebView panel rendering the SVG from c4drill/renderDiagram (the wire shape is already documented in internal/lsp — RenderDiagramParams/RenderDiagramResult — and the intended wiring is sketched in zed-extension/README.md).
2. Debounced auto-refresh on document change (~200 ms), matching feat: VS Code extension with syntax highlighting, autocomplete, and syntax checking for TOML and C4D #27's webview and feat: JetBrains IDE plugin with syntax highlighting, autocomplete, and syntax checking for TOML and C4D #29's tool window.
3. Intercept the SVG's internal drill-down links (relative .svg URLs encoding the target unit path via ComputeExploreURL) → re-issue c4drill/renderDiagram for that path; breadcrumbs; external http(s) → system browser.
4. Parse/validate failures render CLI-identical messages instead of a stale diagram.

Acceptance

• Preview panel parity with the VS Code extension's webview: auto-refresh, drill-through, breadcrumbs, controls (expanded set, legend), error states.
• Update zed-extension/README.md to move the preview from "fallback" to first-class.