Validate combined scroll state and delta handling

[KulmanD]

What changed

• combine the accepted Reset scroll smoothing state between independent scroll sequences #24 scroll-sequence reset and Preserve all CGEvent scroll delta representations #25 delta-field consistency implementations
• retain both isolated test suites and add two assertions that exercise both behaviors together
• add a development-only runner for repeatable strict-idle candidate measurements
• record the static interaction cost and deferred maintainer gates

Why

Both accepted candidates modify the same scroll callback. This branch proves their code and state transitions coexist before either runtime change is promoted.

Validation

• swift build
• swift build -c release
• GitHub Actions: 43 tests, 0 failures
• shell syntax checks
• source sharing archive excludes tests and scripts/performance/
• no XCTest/Testing linkage
• no timers, background threads, retained event history, or reposted scroll events
• measurement-runner smoke test stopped the worker and preserved user config

Performance and remaining checks

The combined helper is 433624 bytes, 1072 bytes over baseline and 48 bytes above the sum of the isolated deltas. Long repeated idle, combined fixed-event active cost, and physical MX Master/trackpad checks are deliberately deferred to the maintainer using:

bash scripts/performance/measure-candidate.sh --label scroll-processing-integration

This PR remains draft and must not merge until those deferred interaction gates are recorded. Related: #24, #25.

[KulmanD]

Superseded by draft PR #40, which contains this change, its performance evidence, the complete accepted integration, and passing final CI. Closing this draft without merging avoids duplicate commits. The branch is intentionally retained for isolated comparison and deferred measurements.