Add multi-change comparison timeline (log-scale)

[VeryComplexAndLongName]

Summary

• Third and final change building the "change timeline" feature (data layer in Add change-timeline data layer (git-derived dates) #91, single-change view in Add single-change timeline view (webui + VS Code extension) #92).
• New global command openspec-ui.showAllChangesTimeline (Command Palette, multi-select showQuickPick over active+archived changes) and a "Compare changes" mode in the standalone Timeline tab (date-range picker + multi-select): both show several changes as parallel lanes on a shared, log-scaled time axis.
• logPosition (timeline-scale.ts): log1p-from-range-start, validated against this repository's own real archived-change timestamps in a throwaway Playwright sketch before implementation — "from start" spreads a dense squash-merge cluster into readable detail; the opposite direction (from range end) compressed the same cluster tighter than a plain linear scale, defeating the point.
• Reuses Add change-timeline data layer (git-derived dates) #91's getChangeTimelines batch function and Add single-change timeline view (webui + VS Code extension) #92's ChangeTimeline type completely unchanged — no data-layer changes.

Two real gaps found and fixed during this change's own verification

1. Missing CSS entirely for both this change's and Add single-change timeline view (webui + VS Code extension) #92's timeline components — left: X% inline styles do nothing without position: relative/absolute, so the positioned-dots axis (the actual point of this feature) did not render at all. Added .openspec-timeline-*/.openspec-multi-timeline-* rules to shell-ui.ts.
2. archivedDate anchored to midnight could sort an archived change's archive-point before that same day's real created/task timestamps (archiving is chronologically last, but the date has no time-of-day). Anchored to end-of-day instead, in both the component and the extension's computeDefaultRange.

Test plan

• timeline-scale.test.ts, MultiChangeTimelineView.test.tsx (new)
• New commands.test.ts cases: multi-select across active/archived, computed default range, no-selection no-op
• npm run typecheck, npm run lint (including lint:english), npm run test workspace-wide
• npm run package --workspace openspec-ui-vscode — VSIX packages without error
• Real-browser smoke test (Playwright) against dist/timeline.js loading real data for 7 of this repository's own archived changes — correct lane/point counts, zero console errors, and (after the CSS fix) confirmed via computed styles that position/left/transform resolve correctly on each point
• openspec change validate --strict add-multi-change-timeline-view

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