feat(fusion_plating_jobs): fp.job.step blocker_kind/reason/jump_target computes

Plan task P1.2. Reuses _fp_should_block_predecessors so the new compute
stays in sync with the existing can_start logic. Drives the OWL GateViz
component on the tablet — "Can't start yet — Waiting on Step N: X".

Future work: extend with explicit branches for contract_review /
parts_not_received / racking_required / manager_input as those gate
models mature.

Tests not run locally (no fusion_plating mount in odoo-modsdev).
Verify on entech: -u fusion_plating_jobs --test-tags fp_jobs.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

fusion_plating/fusion_plating_jobs/tests/__init__.py

@@ -2,3 +2,5 @@
 from . import test_fp_job_extensions
 from . import test_fp_job_milestone_cascade
 from . import test_qty_received_propagation
+from . import test_display_wo_name
+from . import test_blocker_compute