[jiro-by-nori]

Summary

🤖 Generated with Nori

• Fixes a bug found while verifying Edit coaster track from a selected piece #194 on opencoaster.noriagentic.com. With nothing selected, Previous piece did nothing on a closed circuit. The lookup for "the last piece built" skipped every piece that had a next piece, and in a ring that is every piece.
• With nothing selected, Previous now selects the last piece built (the same piece Demolish last piece removes), on unfinished track and on circuits alike.

Test Plan

• Added to e2e/track-edit-selection.spec.ts: after rebuilding the circuit, press Previous piece and expect Selected piece 7 of 14 (the piece that closed the gap), then Track end. This failed before the fix and passes after it.
• All 8 track-edit e2e combinations pass (WebGL2 and Canvas2D, desktop and Pixel 8).
• typecheck, lint, prettier and src/ui unit tests pass.

Requested in Nori: https://demo.norisessions.com/chat/01a16c13-c814-7295-9955-b11ff875ed14

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