Deterministic timer harness for testing Sub.Every apps

[rorygraves]

Background

Apps using Sub.Every (DigitalClock, SineWaveApp, RenderStressApp) can't be snapshot-tested today — time advances on a real scheduler. Current tests skip them.

Goal

A test harness that drives time virtually — advance by N ticks and observe frames, without real-time delays.

Scope

1. Abstract the scheduler behind a Clock interface.
2. ManualClock implementation that advances only on explicit calls.
3. TuiTestDriver accepts a clock + exposes advanceTime(duration).
4. Retroactively cover DigitalClock and SineWaveApp with snapshot tests.
5. Document in RENDER_PIPELINE.md or a new test-harness doc.

Acceptance

• ManualClock exists and is used by test driver.
• DigitalClock has at least one snapshot test asserting time-advance behaviour.

Part of Cross-cutting — Semantic model & test debt.

[rorygraves]

Implemented in #217 (merged): Clock/ManualClock abstraction, TuiTestDriver.advanceTime, and deterministic snapshot tests for DigitalClock/SineWaveApp. Acceptance criteria met. Two follow-ups filed for gaps surfaced in review: #216 (forward clock through RuntimeCtx wrappers) and #215 (interleave timer ticks with updates during a multi-period advance).