feat(vscode): compare arbitrary revision graphs

[forhappy]

Summary

• replace adjacent-parent-only comparison actions with an explicit baseline revision picker
• compare the selected graph against any loaded revision, including non-adjacent commits
• build a missing selected or baseline graph in place without losing the comparison pair
• keep change counts aligned with the chosen baseline and clear stale comparison results when it changes
• align the query action, baseline picker, and compare action on one shared control baseline
• make the native comparison picker follow VS Code light, dark, and high-contrast themes

Why

The comparison backend already accepted arbitrary revision IDs, but the history UI exposed only parent buttons. That made non-adjacent comparisons impossible even when both graphs were available. Missing graphs also forced users out of the comparison flow.

This change exposes the backend capability directly and keeps graph materialization, comparison, and recovery in one workflow.

User impact

Users can select any loaded commit as a baseline, load older commits through the existing timeline pagination, build either missing graph on demand, and inspect source, graph, and semantic changes between arbitrary revisions.

Screenshots

Light theme

Dark theme

Validation

• npm test -w @compass/viewer — 110 tests passed
• npm test -w crabbuild-compass-vscode — 100 tests passed
• npm test -w @compass/viewer-tests -- history.spec.ts — 14 tests passed
• npm test -w @compass/viewer-tests -- theme.spec.ts — 16 tests passed
• npm run typecheck -w @compass/viewer
• npm run typecheck -w crabbuild-compass-vscode
• graphify update .
• git diff --check

[forhappy]

Updated the revision action layout in 4674ca9:

• aligned Query this revision, the baseline picker, and Compare revisions to the same 28px control baseline
• preserved a clean stacked layout at narrow widths
• added a browser geometry assertion for matching top and bottom coordinates
• refreshed the light and dark screenshots in the PR description

Validation: history.spec.ts now passes all 14 tests, and the viewer typecheck passes.

[forhappy]

CI asset drift is fixed in 90daf8a by regenerating the checked-in viewer bundle and manifest.

Validation:

• node scripts/check_viewer_assets.mjs ✅
• npm run typecheck -w @compass/viewer ✅
• javascript-and-vscode CI job ✅

The remaining matrix jobs are still in progress.