0011. The performance contract: identity, not diffing

Context

A drag re-resolves the file on every pointer move and pushes the result into the player and the DOM. Measured on a 1,000-element project before any work: 5.5 ms per step, with Zod re-parsing every element.

Decision

The resolver guarantees identity for what did not change: the overlay keeps every untouched node object, parsed props are cached per raw props object, intrinsic timing is cached per node, and a placement whose timing, props, errors and children are equal to the previous resolve is the same object. Consumers memoise on identity: the DOM layer skips elements whose placement is the same object, decoders are keyed by asset and take a new trim in place, and a paused preview paints at once and fetches the exact frame after the gesture is quiet. A benchmark (pnpm --filter @cutcraft/kit bench) and tests guard the identities and a loose budget.

Consequences

• About 1 µs per element per step (1.0–1.3 ms on 1,007 nodes).
• Anything that computes new placement objects for unchanged elements breaks the contract; the perf test fails.
• Per-track incremental placement is a possible next step, not taken: it needs anchor dependency tracking.