fix(codec): preserve sparse array slots in state previews

[shuaiyafei]

Summary

Materialize array preview entries with Array.from so sparse slots are represented as explicit encoded undefined values.

Previously, slice(...).map(...) retained holes in the preview array. A JSON transport then converted those holes to null, which is not a ValueEntry and cannot be read as preview.key / preview.value by the state inspector.

For example:

const value: unknown[] = []
value.length = 3
value[1] = 'visible'

JSON.parse(JSON.stringify(encodeValue(value))).preview
// Previously: [null, { key: '1', value: ... }, null]

The encoder now returns a dense preview with the original indices, without filling or mutating the inspected source array.

Regression Coverage

• Mixed sparse slots and values survive a JSON round trip as valid preview entries.
• An offset/limited preview retains the correct original array indices.
• The inspected array keeps its original holes.

Both regression tests failed before the encoder change and pass with the fix.

Validation

• pnpm lint
• pnpm test: 407 tests passed across 67 files
• pnpm typecheck
• pnpm build

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commit: 4179e94