feat(equipment): register_fixture rejects Decommissioned bound assets

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Summary

Adds a single cross-aggregate guard at register_fixture time: every Asset referenced by slot_asset_bindings must NOT be Decommissioned. Mirrors the sibling AssetCannotAttachToFixtureError precondition at attach-time and rejects at register-time so the operator doesn't register a Fixture that would inevitably fail later at attach_asset_to_fixture (Fixture is single-event-genesis and cannot be amended).

• New FixtureAssetNotAttachableError carries the sorted-first offending asset_id and current lifecycle string, mirroring the FixtureAssetNotFoundError deterministic-error precedent in the same decider.
• RegisterFixtureContext gains a lifecycle_by_asset_id dict populated from the existing per-Asset load_asset gather (no extra round-trip, no new projection). Default empty dict means decider-only unit tests that exercise other invariants leave the guard inactive (mirrors family_ids_by_asset_id's relaxed default).
• Decider guard ordering: existence (FixtureAssetNotFoundError) → NEW lifecycle (FixtureAssetNotAttachableError) → unknown-slot → cardinality → family-mismatch → param-overrides. Operator sees the most actionable error first.
• Route + OpenAPI 409 description list the new cause; routes.py wires the new error into the existing _handle_cannot_transition family alongside its Asset-BC mirror.

Zero existing tests break because every existing happy-path test uses Active assets.

Closes INV-5 from the Fixture+Mount+Asset alignment plan; first half of Slice 3 (INV-4 orphan-binding guard deferred to Slice 3b to keep this slice ripple-free). Slice 3a in the Option A sequence.

Test plan

• Two new decider unit tests (guard fires with sorted-first determinism + empty-dict short-circuit) — green locally
• One integration test exercising the full handler stack end-to-end against Postgres (register_asset → decommission_asset → register_fixture → FixtureAssetNotAttachableError) — green locally
• Existing register_fixture decider + endpoint suite (19 tests) — green locally
• CI confirms full suite + architecture fitnesses pass

🤖 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/equipment
routes.py
apps/api/src/cora/equipment/aggregates/assembly
state.py
apps/api/src/cora/equipment/features/register_fixture
context.py
decider.py
handler.py
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