Keep the reference grid fixed for each viewer dataset

[marpaia]

Closes #20.

Growing or translating colonies currently rescale and move the ground grid on every frame. Initialize the grid once from finite device geometry or initial colony bounds, preserve it through empty frames and same-dataset reset/seek, and reinitialize only when another dataset opens. Camera Fit remains independent.

Establish an explicit dataset lifecycle and retain channel/slice preferences when data temporarily disappears. Document finite-device priority, the fixed minimum extent, ignored infinite planes, and the initially empty dataset fallback.

Validation: 36 Vitest tests, TypeScript/build/format checks, and real Chromium 153 regression covering grid matrices and projected coordinates, orbit/pan/zoom/Fit, growth/movement/division/removal/reset, missing channels/grids, axis round trips, and opening another file. Screenshots were inspected to confirm a stationary grid during growth. The committed viewer/browser/reference-grid.mjs reproduces browser checks with Playwright and a Vite server.

This independent PR targets master. It supplies the dataset lifecycle prerequisite for #15, #16, and #23. Reversed-time frame transitions and the actual #16 replay UI both passed combined acceptance. No native engine changes.

Independent review is complete against the original acceptance criteria and intended review base. The review report records per-criterion evidence, both corrected campaign findings, exact tested heads, landing order and platform limits.

Merged into master in the approved campaign order: #26, #27, #28, #31, #34, #36, #29, #30, #33, #35, #37, #32. The final tree preserves the reviewed implementation and integration regressions. Prerequisites were integrated with merge commits, and the frozen review bases were retained.

Landing validation: 534 Python tests passed (57 environment-dependent skips), 115 viewer tests passed, Ruff lint, strict type checks, viewer build/format checks, and actual Chromium feature/integration checks passed. Pyright retains the existing native-extension source warning. A Windows-only documentation link-test encoding failure was reproduced and fixed with an explicit UTF-8 read; the corrected final PR heads passed Windows CLI/shutdown checks and CUDA compilation. NVIDIA runtime execution remains unverified.