0003. Editor state is the file plus a patch log; actions have four faces

Context

A drag produces a hundred updates a second. Rewriting the file on each one would be slow, would churn the model's view and would make undo a text-diff problem. Yet the file must end up holding the result (0001).

Decision

Editor state is { file, patches }. A gesture dispatches actions. Each action handler has four faces: validate against the resolved state, apply as patches appended to a log, invert computed from the state before, and fold, which turns the net patches into source edits. The resolver applies the log's net effect over the file on every change (memoised per log version). History groups a gesture into one undo step and auto-groups rapid same-target updates. At a sync point the log is folded into the file: one changed token per touched prop, printed elements for insertions, baked literals for anything generated.

Consequences

• The live path never touches text; a drag step costs about a microsecond per element (0011).
• Every action has an inverse, so undo re-runs actions instead of restoring snapshots, and the history is a readable list of what the user did.
• The fold is the only place where "edit the code" is confronted, and it is a token edit, not a transpile.
• Policy: the fold runs when a history group closes, so the file is at most one gesture behind the preview (decided in 0012 after studying Diffusion Studio, whose debounced writer can write a drag several times).

Alternatives

Elements as reducers over a runtime document with no write-back: see 0001. Operational transforms on text: undo and validation would have to be re-derived from diffs.