Land shared renderer and session preservation on master

[happykhan]

Summary

• carry the already-reviewed shared Canvas/SVG plot scene from Centralize Canvas and SVG plot geometry #43 onto master
• carry the already-reviewed hidden-GC session preservation from Preserve hidden GC data in saved sessions #44 onto master
• include the same nanoid 3.3.18 lock correction as Update nanoid to 3.3.18 #45 so this integration head passes CI independently
• make the architecture guard portable on GitHub runners without requiring ripgrep

Why this PR exists

#43 was merged into the #42 feature branch, and #44 was merged into the #43 feature branch. GitHub therefore marked both PRs merged, but neither merge reached master. This catch-up branch has been rebuilt cleanly from current master to avoid the conflicts caused by the earlier stacked/squash history.

Merge order

Prefer merging #45 first, then #46. Once #45 is on master, GitHub will remove the already-landed nanoid lock change from this PR's effective diff. However, #46 is also self-contained and verified if merged first.

Verification

• npm run verify
• dependency audit passed with zero vulnerabilities
• 181 unit tests passed
• production build passed
• 25 production browser tests passed
• development-runtime browser test passed
• GitHub CI and Cloudflare Pages passed
• live Cloudflare previews for Centralize Canvas and SVG plot geometry #43 and Preserve hidden GC data in saved sessions #44 were exercised successfully

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