[jiajic]

What changes

• Docking. Chart panels dock beside the spatial view as tabs and splits, can be dragged, closed, and added back from a + menu in each tab bar. The spatial view is not a panel and stays where it is. Default: scatter above histogram (tabs when the dock is short).
• Stable panel ids. Each panel's id names its dock slot, its selection source (scatter / scatter:<id>, panel:<id>) and its data file, so moving a panel changes nothing else.
• R API. push_panel(spec, data, id = NULL, title = NULL, where = c("tab", "right", "below"), ref = NULL) returns the id; pushing to an existing id replaces that chart in place; works before a browser connects. pane = is dropped. New: remove_panel(id), panels(). describe() and state() list panels.
• Restore on reload. zustand's persist middleware sends the config (layers, images, genes, colouring, annotations, tool settings, layout) to R on change, merged over 300 ms. R keeps it verbatim as JSON text and sends it back first when a page connects (restore), followed by metadata, rules and the pushed panels it holds. Protocol 2 → 3.
• Brushes from rules. The histogram and pushed charts draw their brush from their own rule (a brush, R, or a reload), so a restored range can still be dragged. A pushed chart's ids rule keeps params.brush.
• Background tabs skip selection updates and catch up when shown.

Choices

• dockview-core 4.13.1, pinned (adr/0009). 8.8.3 bundles every feature module (368 KB / 88 KB gz JS) and its module build ships no stylesheet. golden-layout has had no release since Feb 2023. Bundle cost: main chunk 2,076 → 2,245 KB (+41 KB gz), plus a 37 KB stylesheet (4 KB gz).
• R holds the saved config, not browser storage: R outlives the page, re-pushes the panels the layout names, and a save to file (Save the viewer state to a file and restore it in a later session #21) can start from its copy.

Not in this PR

• The camera position isn't saved; a reload re-fits the view (Save the viewer state to a file and restore it in a later session #21).
• Save/load of the state to a file: Save the viewer state to a file and restore it in a later session #21.

Tests

• vitest 52/52; tsc clean.
• R suite passes, including new socket tests (config round trip, held panels, panel_closed) and e2e tests (placement, reload restoring layout, panels, settings, selection and draggable brushes).
• The flaky "rules are visible" e2e test (failed 4–5 of 8 runs, on dev too) clicked a rule's × before the page had received the rule; it now waits. 8/8 runs pass.
• R CMD check --no-tests: 0 errors, 0 warnings, 1 note (R6, grDevices and later listed in Imports but not imported).

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