docs: add 3d euler beam phase

phases/euler-beam-3d/step0.md

@@ -0,0 +1,63 @@
+# Step 0: requirements-baseline
+
+## Read These Files First
+
+Read the following files before editing:
+
+- `/AGENTS.md`
+- `/docs/PRD.md`
+- `/docs/ARCHITECTURE.md`
+- `/docs/ADR.md`
+- `/docs/requirements/README.md`
+
+## Task
+
+Create `/docs/requirements/euler-beam-3d.md` for a kernel-first 3D Euler-Bernoulli beam feature.
+
+Scope the first implementable increment narrowly:
+
+- two-node, straight, prismatic, small-displacement 3D Euler-Bernoulli beam element
+- six mechanical DOFs per node in this order: `ux, uy, uz, rx, ry, rz`
+- linear elastic section constants: `E`, `G`, `A`, `J`, `Iy`, `Iz`
+- local and global 12x12 stiffness matrix support
+- local and global element end-force recovery from nodal displacement vectors
+- no shear deformation, warping, end releases, offsets, distributed loads, mass matrix, geometric stiffness, nonlinear kinematics, dynamics, or thermal coupling in this increment
+- no Abaqus reference CSV generation in this increment
+- no full Abaqus compatibility claim
+
+The document must contain:
+
+- metadata: `feature_id: euler-beam-3d`, owner agent, status
+- explicit assumptions and non-goals
+- `must` requirements with stable IDs such as `EB3D-REQ-001`
+- verification quantities: displacement DOFs, reactions/end forces, stiffness symmetry, rigid body modes, local/global transformation consistency
+- acceptance criteria that distinguish kernel completion from full solver release readiness
+- open issues for parser integration, reference artifact availability, and full end-to-end assembly
+
+Do not create C++ files in this step.
+
+## Tests To Write First
+
+- No C++ test is required in this documentation-only step.
+- Still run the harness validation commands in the acceptance criteria.
+
+## Acceptance Criteria
+
+```powershell
+python -m unittest discover -s scripts -p "test_*.py"
+python scripts/validate_workspace.py
+```
+
+## Verification Notes
+
+1. Confirm the requirements do not claim full Abaqus compatibility.
+2. Confirm each `must` requirement has an acceptance criterion or a downstream verification hook.
+3. Update `phases/euler-beam-3d/index.json` step 0:
+   - success: `"status": "completed"`, `"summary": "3D Euler beam kernel requirements baseline added"`
+   - failure after retries: `"status": "error"`, `"error_message": "<specific error>"`
+   - blocked: `"status": "blocked"`, `"blocked_reason": "<specific reason>"`
+
+## Forbidden
+
+- Do not create or modify reference CSV files.
+- Do not modify source or test files.