fix(vscode): restore a diagram panel across an extension update

[devin-ai-integration]

What and why

A diagram panel left open across an extension update restored blank after the reload: no drawing, an empty view selector. Closing and reopening it recovered, and a same-version reload was never affected.

VS Code restores a serialized webview panel with the WebviewOptions it was created with, so its localResourceRoots still name <old extension dir>/dist. After installing another version the extension (and its bundled webview.js) lives under a differently named directory, and the script asWebviewUri points at is outside the restored roots, so the webview blocks it.

DiagramPanel now sets webview.options from the installed version's extensionUri before setting the page; create and the serializer's adopt share one webviewOptions(extensionUri):

// DiagramPanel constructor
this.panel.webview.options = webviewOptions(extensionUri);
this.panel.webview.html = html(this.panel.webview, extensionUri, docURI, selected);

With the nightly snapshot now shipping a .vsix whose version changes every night (#371), this becomes the ordinary update path, so it is worth fixing before users hit it.

How it was verified

• npm run typecheck, npm test (159 tests), npm run build -- --production in editors/vscode; python3 scripts/changelog.py check.
• The serializer path needs a real VS Code (the unit tests do not import vscode), so it is verified by hand: install one version of the .vsix, open a diagram, install a package stamped with another version, reload the window — the restored panel draws.

Checklist

• make test and make lint pass locally (extension typecheck/tests/build; no Go change)
• Tests added or updated for the change (webview restoration is not unit-testable without the vscode host; verified in VS Code)
• Documentation extended where it already covers the surface (no documented surface changes)
• Changelog entry added as changes/unreleased/<slug>.<section>.md, not as an edit to CHANGELOG.md
• baselines regenerated and make docs-counts run if a gate count moved (none moved)
• No internal work-item labels (waves, slices, F4, K5) in the body, docs, or changelog

[devin-ai-integration]

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[devin-ai-integration]

Verified in real VS Code (Linux) at d144167

The restoration scenario that failed before this change now passes:

• Installed 0.1.0-nightly-20260917-aaaaaaa and opened a rendered diagram.
• With that panel open, installed 0.1.0-nightly-20260918-bbbbbbb and reloaded the window: nodes visible, view dropdown populated, no close/reopen needed.
• Repeated the open-panel transition to stable 0.1.0: same result.
• Extensions view showed each exact installed version.

nightly → nightly reload	nightly → stable reload

Regression checks

Same-version reload and Alt+D close/reopen render normally. Introducing and fixing an unresolved reference (Enginex) updated diagnostics and the diagram label; Problems ended empty. Linux only.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.