#3710 refactor(capture-kit): move screenshot-diff beside its pixel worker and overlays

Summary

Moves the seven src/screenshot-diff/ modules (961 LOC) and their test into packages/capture-kit/src/, next to screenshot-diff-pixels, the PNG worker and the screenshot overlays they already depended on. Every outgoing edge already targeted kernel/capture-kit, so no new package or dependency is needed. The screenshot-diff layering zone is removed.

• Consumers (src/cli/commands/screenshot.ts, src/commands/capture/runtime/diff-screenshot.ts, src/commands/output/diff.ts) now import @agent-device/capture-kit/screenshot-diff, /screenshot-diff-regions and /screenshot-diff-overlay-matches.
• The test uses capture-kit's local tmp-dir.fixtures.ts (capture-kit cannot depend on test-kit, which depends on it).
• 15 files; ADR 0019 inventory updated. No behavior change.

Validation

Tested at 05c664543:

• pnpm check:affected --run passed (layering, production-exports, package).
• vitest run packages/capture-kit/src/screenshot-diff.test.ts: 22/22.
• git diff -M90% shows renames with only import-specifier edits.
• Fresh-context adversarial review: no findings.

Not run: device lanes (no runtime path change). CI pending.