docs: add 3d euler beam phase

phases/euler-beam-3d/step3.md

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+# Step 3: numerical-review
+
+## Read These Files First
+
+Read the following files before editing:
+
+- `/AGENTS.md`
+- `/docs/PRD.md`
+- `/docs/ARCHITECTURE.md`
+- `/docs/ADR.md`
+- `/docs/requirements/euler-beam-3d.md`
+- `/docs/research/euler-beam-3d-research.md`
+- `/docs/formulations/euler-beam-3d-formulation.md`
+- `/docs/numerical-reviews/README.md`
+
+## Task
+
+Create `/docs/numerical-reviews/euler-beam-3d-review.md`.
+
+Review the formulation for implementation readiness. The review must include:
+
+- dimension and units check for each stiffness coefficient
+- symmetry and rigid body mode expectations
+- local axis construction risks
+- positive semi-definite local stiffness expectation before constraints
+- test obligations before production code
+- known numerical limits for slender beams and very small section constants
+- pass/fail verdict for kernel implementation planning
+
+If the formulation is not ready, mark the document status as `needs-upstream-decision` and update this phase step as blocked with a concrete reason.
+
+Do not create C++ files in this step.
+
+## Tests To Write First
+
+- No C++ test is required in this documentation-only step.
+
+## Acceptance Criteria
+
+```powershell
+python -m unittest discover -s scripts -p "test_*.py"
+python scripts/validate_workspace.py
+```
+
+## Verification Notes
+
+1. Confirm the review does not change requirements or formulation.
+2. Confirm implementation-owned risks are turned into concrete tests.
+3. Update `phases/euler-beam-3d/index.json` step 3:
+   - success: `"status": "completed"`, `"summary": "3D Euler beam numerical review added"`
+   - failure after retries: `"status": "error"`, `"error_message": "<specific error>"`
+   - blocked: `"status": "blocked"`, `"blocked_reason": "<specific reason>"`
+
+## Forbidden
+
+- Do not modify requirements, formulation, source, tests, or reference artifacts.