Test coverage: render-mode transitions (normal/lyrics/viz) and canvas context-type swap

[carochacs]

Part of the test-coverage epic (#18).

Scope:

• Mode transitions: normal highway ↔ lyrics pane ↔ viz renderer, verifying mutual exclusivity (entering one mode exits the others cleanly).
• recreatePanelHighway() — discarding the previous highway instance before a new renderer takes over.
• Canvas context-type lock/swap behavior when switching 2D ↔ WebGL viz renderers mid-session.
• Viz-mode arrangement switching (panel.select.onchange in-place viz switch branch).
• Per-panel viz popover lifecycle: _showVizControls/_hideVizControls, rebuild-on-open behavior.

[pullfrog]

  ^{｜ Build this ➔ ｜ Make a plan ➔}

[carochacs]

Partial progress via #61 (open): covers _showVizControls/_hideVizControls/_closeAllVizPopovers (per-panel viz popover lifecycle) and recreatePanelHighway (the highway-discard mechanism the mode transitions and context-type swap build on — old-highway stop, canvas replacement, state transfer, preInstallRenderer-before-init() ordering).

Still open: the full enterVizMode/exitVizMode/enterLyricsMode/exitLyricsMode mode-transition flows, viz-mode arrangement switching (panel.select.onchange's in-place viz-switch branch), and 2D↔WebGL context-type swap behavior — these call populateSelect/vizFactory resolution on top of recreatePanelHighway and need more harness plumbing.

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Generated by Claude Code

[carochacs]

#61 merged. Still open on this issue: the full enterVizMode/exitVizMode/enterLyricsMode/exitLyricsMode mode-transition flows, viz-mode arrangement switching, and 2D↔WebGL context-type swap behavior — these build on recreatePanelHighway (now covered) but need populateSelect/vizFactory resolution plumbing that wasn't in scope for this PR.

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Generated by Claude Code