Delete docs/v0.3.0-design.md and strip all references to it

The design doc and its followups doc are no longer needed as a live
reference now that the v0.3.0 redesign is implemented — comments and
docstrings across the codebase cited it extensively (file path, "design
doc §X.Y", "decision N", or bare "§X.Y" section numbers) as design
rationale. Removed docs/ and edited every citing comment/docstring to
drop the now-dangling reference while keeping the substantive
explanation next to it. CLAUDE.md's v0.3.0 roadmap bullet loses its
trailing pointer to the deleted file.

Verified: no remaining "docs/v0.3.0", "design doc", "decision N", or
"§N.N" references (repo-wide grep); ruff and ty clean; full test suite
on the heaviest-touched modules (network, sample, rollout, migration,
config, train) passes.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/test_network.py

@@ -84,7 +84,7 @@ def test_stage1_model_gradients_flow():
 
 def test_stage1_model_no_n_sec_head_by_default():
     """Fresh v0.3.0 construction (no n_sec_head_k_max) has no n_sec head —
-    decision 1 (docs/v0.3.0-design.md §2) moves it to stage 2."""
+    it moves to stage 2."""
     model = Stage1Model(
         pdg_vocab=3, mat_vocab=2, particle_cfg=PARTICLE_CFG, material_cfg=MATERIAL_CFG
     )
@@ -206,7 +206,7 @@ def test_condition_encoder_onehot_is_a_true_one_hot_vector():
     assert torch.all(pdg_e.sum(dim=-1) == 1.0)
 
 
-# --- Stage2OneShot particle_type architecture (docs/v0.3.0-design.md decision 2) --
+# --- Stage2OneShot particle_type architecture --------------------------------
 
 
 def _build_stage2(target: str, generator: str, emb_dim: int = 6) -> Stage2OneShot:
@@ -300,7 +300,7 @@ def test_stage2_oneshot_forward_shape_onehot_flow_excludes_type():
     assert out.shape == (B, k_max * CONT_SLOT_DIM)
 
 
-# --- MarkovHistory (docs/v0.3.0-design.md §6.2) -----------------------------
+# --- MarkovHistory -----------------------------------------------------------
 
 
 def test_markov_history_shape():
@@ -314,8 +314,8 @@ def test_markov_history_shape():
 
 def test_markov_history_uses_start_vector_when_no_prev():
     """Slot 0's own raw feature must be ignored — a learned start vector is
-    substituted there instead (a reasonable default not specified by the
-    design doc, see Stage2Autoregressive's docstring)."""
+    substituted there instead (a reasonable default, see
+    Stage2Autoregressive's docstring)."""
     hist = MarkovHistory(in_dim=4, out_dim=6)
     B, K = 2, 3
     has_prev = (torch.arange(K) >= 1).unsqueeze(0).expand(B, -1)
@@ -328,7 +328,7 @@ def test_markov_history_uses_start_vector_when_no_prev():
     assert torch.allclose(out_a[:, 1:], out_b[:, 1:])
 
 
-# --- AttentionHistory (docs/v0.3.0-design.md §6.2, v0.3.0 step 7) ----------
+# --- AttentionHistory (v0.3.0 step 7) ---------------------------------------
 
 
 def test_attention_history_shape():
@@ -392,7 +392,7 @@ def test_attention_history_step_matches_forward():
     assert torch.allclose(stepped, expected, atol=1e-5)
 
 
-# --- Stage2Autoregressive (docs/v0.3.0-design.md §6, v0.3.0 step 5) ---------
+# --- Stage2Autoregressive (v0.3.0 step 5) -----------------------------------
 
 
 def _build_stage2_ar(
@@ -671,8 +671,8 @@ def test_build_models_share_stages_true_shared_params_are_in_both_stage_paramete
     """The shared encoder's parameters must actually appear in both stages'
     own `.parameters()` — that's what makes each stage's independent
     optimizer include (and update) them, which is the actual mechanism behind
-    "shared weights, forced common representation" (docs/v0.3.0-design.md
-    §3.1), not just object identity on `.cond_enc`."""
+    "shared weights, forced common representation", not just object identity
+    on `.cond_enc`."""
     built = build_models(_minimal_model_config(share_stages=True))
     stage1, stage2 = built["stage1"], built["stage2"]
     assert stage1 is not None and stage2 is not None