feat(derive): Add Signal API

[clabby]

Overview

Adds the notion of a Signal to the pipeline, with a FlushChannel variant for Holocene.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 19 lines in your changes missing coverage. Please review.

Project coverage is 81.1%. Comparing base (7302685) to head (aa315f2).
Report is 3 commits behind head on main.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
crates/derive/src/pipeline/core.rs	0.0%	19 Missing ⚠️

Additional details and impacted files

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