JS frontend coding style

The frontend (py/pytanga/viz/templates/) must be written so its logic can be unit-tested without a browser.

Rules

1. Pure logic goes in DOM-free modules. Any logic that does not need document, window, or three — reconciliation, sizing math, shader assembly, serialization helpers — lives in a .js module that imports none of those. Reference: templates/views/reconcile.js (layout reconciliation / orphan-map accounting).
2. Unit-test pure modules with node --test. Every pure module has a sibling *.test.mjs under js/dev/tests/, run with node --test.
3. Keep DOM/WebGL glue thin. view.js / three-view.js / viewer.js / build.js stay thin adapters over the pure modules; their behaviour is validated by the Playwright smoke (js/dev/tests/reconcile-smoke.mjs), not by unit tests.
4. One live-view registry. The frontend keeps a single _viewRegistry keyed by stable view id (see viz-architecture.md); do not add parallel id-indexed maps or orphan registries.

Example

templates/views/reconcile.js exports the pure collectNodeTypes + planReconciliation; build.js consumes them, and js/dev/tests/reconcile.test.mjs asserts the reuse/orphan semantics (reuse by id and type, create new ids, orphan unclaimed views).