Extract the signals-engine file

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Problem: This is the single largest, highest-fan-in file in the entire candidate set — thousands of lines, over a hundred exports, more than twenty external consumers across nearly every part of the codebase. It's the biggest blast-radius item in this whole initiative and should be done alone, last, not combined with any other phase.

Area: ORB / gittensory-engine

Proposal: Move the file into gittensory-engine, leaving a temporary full re-export shim in its old location so none of its many consumers need an import-path change in the same PR. Follow the same playbook already recommended for the in-flight queue-decomposition work: one direction of movement per PR, verified against the coverage/rename tooling (per the first issue in this phase) before doing it for real.

Deliverables:

• The file moved, with a working full re-export shim, all existing consumers unaffected.

Acceptance criteria:

• Every one of this file's known external consumers continues to work with zero import-path changes required in this PR.
• Full existing test coverage for this file passes unchanged.

Boundaries:

• Do not combine this move with any other refactor, cleanup, or behavior change in the same PR — the blast radius alone is enough to manage carefully.

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JSO-3544