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perf(render): materialize expandable children lazily (#26)

* perf(render): materialize expandable children lazily

DataRender eagerly built an Array<[key, value]> tuple array for every
node, but a collapsed ExpandableObject only ever needs its child *count*.
Under collapseAllNested on a 100k-element array that is 100k throwaway
tuple allocations (and 100k child-value reads) at mount.

Move materialization into ExpandableObject, gated behind a hasMaterialized
latch: a node that is never opened never allocates its tuples or reads its
values; one opened once keeps them (no re-read on re-expand). Beats the
upstream react-json-view-lite, which rebuilds this array on every render.

- DataRender passes the resolved isArray bit down instead of the tuples,
  keeping the array-vs-object dispatch in the one component that owns it.
- ExpandableObject shares a single objectKeys derivation between the count
  and the tuple builder (was enumerating keys twice on expanded objects).
- untrack the shouldExpandNode args in the expansion effect so an ancestor
  re-render that mutates level/value/field can't rerun it and clobber the
  user's manual expand/collapse (surfaced by CodeRabbit).

Adds unit tests asserting zero child-value reads while collapsed and no
re-read on re-expand, plus a /test/lazy-children visual harness with a
live allocation counter and an e2e test.

Closes #21
Refs #22

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* docs: add TokenMaxing badge to README

Add the TokenMaxing "AI tokens used building this repo" badge as the
third badge in the header row.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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