feat(flow-schema): add flow version diff

[wavekat-eason]

What

Adds diffFlows(before, after) to @wavekat/flow-schema — a structural, authoring-only diff between two flows answering "what changed since the last publish?". TS-only, like mutate.ts (the engine never diffs, so no Rust twin).

Motivating consumer: the platform flow editor wants to highlight which nodes have unpublished changes and show, per node, what's different. Today the only signal is the server's byte-compare draftDirty boolean, which carries no per-node info and false-positives on formatting/stamp churn.

Shape

diffFlows(before, after) -> FlowDiff {
  nodes: Record<id, { status: 'added'|'removed'|'modified'|'unchanged', kind, changes }>
  added / removed / modified: string[]     // sorted
  entryChanged, entryBefore, entryAfter
  changed: boolean
}

Each modified node carries FieldChange[] with dot-paths — top-level fields (prompt, timeout_secs), per-exit (exits.no_answer), per-menu-option (options.1), and a synthetic entry when the start marker moves. Raw before/after values only; labels/colours stay with consumers.

Design

• Structural, not textual — compares decoded Flow objects, so comment/key-order churn and the flow-level stamps publish rewrites (version, id, name, ui) never register.
• Entry move shows on both nodes — old and new start node each get an entry change, so a UI highlights exactly what visually moved.
• Object keys compared order-insensitively; arrays (schedule ranges, exceptions) order-sensitively.

Tests

test/diff.test.ts — 11 cases: added / removed / modified, exit rewire, menu option relabel, prompt change, entry move, structural-equality (comment + key reorder), stamp-ignore, added optional field. Full package suite: 99/99 green; typecheck + build clean, so prepublishOnly passes.

Release

Ships as @wavekat/flow-schema 0.0.8 via release-please on merge. The platform then bumps its dep and wires the presentation layer.

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