Fold the genesis Decision link into Run state, not just the event

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Summary

• RunStarted.decided_by_decision_id was fully wired end-to-end (command, decider, REST route, MCP tool) but the evolver's genesis arm never destructured it, so it silently dropped on every run, on every replay, including the D6 fidelity tool's own refold. Verified directly with evolve() before touching anything: the value went in and never came back out.
• Adds Run.started_by_decision_id (deliberately not decided_by_decision_id: eight sibling Run events already use that name for a different, per-transition sense; Run.last_adjusted_by already sets the precedent inside this same aggregate for renaming a field when the folded scope changes).
• Threaded through all 12 evolve() match arms; surfaced in record_fidelity_check's _render_run_state; closes test_2bm_energy_change's own long-standing "linkage is not first-class" watch item by actually wiring and asserting it.
• Adds an AST fitness test covering both ways a field can silently vanish: a non-genesis arm dropping prior.<field>, and the genesis arm never setting the field at all (the shape this bug actually was — the existing test_procedure_evolver_carry_forward.py precedent's genesis exemption would not have caught it).

Test plan

• New unit tests: genesis fold sets the field, default is None, survives every parametrized transition path (tests/unit/run/test_run_evolver.py)
• New AST fitness tests pass, and a mutation test (reintroducing the exact original bug) confirms they fail loudly with a clear message
• test_2bm_energy_change.py integration scenario updated to actually pass decided_by_decision_id and assert it survives onto the folded Run
• Full unit + architecture suite: 37,060 passed, 0 failed
• pyright, ruff check, ruff format clean
• Naming review (naming-r3-reviewer) on both the original name and the rename, both passed
• Independent gate review (2 lenses) on correctness and completeness/blast-radius

🤖 Generated with Claude Code

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Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
apps/api/src/cora/run/aggregates/run
events.py
state.py
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