fix: a tree playground page, and the two bugs building it exposed

[yaotzin1]

What and why

Standing instruction: always update the examples. 0.3.0 shipped tree data, a bubble menu and inline
editing with tests, docs and a spec, and no way to click any of it.

Building the page found two defects that 269 tests had missed, because a test constructs a
component once and an application changes props.

The page

examples/playground/tree.html, over three shapes: nested children, flat rows with two parents,
and lazy children. Bubble menu, inline editing on three columns with different rules, a language
switcher, a "refuse every edit" toggle for watching rollback, and live nested set counters that
make the model legible. Eight nodes for six distinct rows is what "one row, two placements" looks
like.

examples/react-remote/App.tsx gains a tree section, so the tree API is type-checked with the rest.

The bugs

A changed dataSource prop never reached the engine. The guard compared the new prop against a
helper that reads the current props, so it compared the prop against itself, which is always equal,
and setDataSource was never called. Present since 0.1.0.

Changing the tree's input shape left every row a root. The controller chose between nested
children and parent references when it was built. It now asks on every normalise, which is better
than the rebuild I tried first: that cascaded, because plugins are fixed at engine creation, so the
engine kept a reference to a controller that had just been destroyed.

Expanding a node with children present fetched an empty list and replaced them. Fallout of the
fix above, since loadChildren became always-supplied. Loading now runs only for a node that
declared children and has none.

Regression tests for all three.

Semver classification

• patch — no change to the public surface
• minor
• major

TreeControllerOptions gained an optional mode, which is additive.

Verification

Test Files  15 passed (15)
     Tests  271 passed (271)

Test Files   1 passed (1)      [smoke, against dist/]
     Tests  12 passed (12)

typecheck      exit 0
lint           exit 0
check-exports  the published package resolves cleanly.

Driven in a browser: switching to "Flat, with two parents" shows Shared.pdf under both folders,
expanding one leaves the other shut, and the bubble menu hides "Add child" on a file.

Self-review

1. Boundary and layering — no layer moved. The controller now answers a question per call instead of at construction, which removed a rebuild and the destroyed-reference cascade behind it.
2. The local/remote seam — the dataSource swap fix is exactly this seam working as documented for the first time.
3. Public surface and semver — patch, one additive optional option.
4. Accessibility and i18n — unchanged; the new page exercises the existing keyboard routes and the language switcher.
5. Supply chain and packaging — no dependency. examples/ stays out of the tarball.
6. Honest output — the page shows the nested set counters rather than describing them, and the "refuse every edit" toggle makes rollback visible instead of claimed.
7. Verification — the gates above, plus CI now boots the new page.

Documentation

• CHANGELOG.md under Unreleased, naming both bugs and that the examples found them
• examples/playground/README.md gains the page and four things to try
• specs/tree-data/tasks.md gains an Examples section
• .agents/rules/agent_execution_standards.md makes it a rule, so it is planned at stage 4
  rather than remembered
• workflow.ai.yml not affected; the sync scripts still pass with --check