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_train.py

@@ -90,7 +90,7 @@ def test_wandb_run_config_handles_missing_model_config():
     assert wcfg["model_config"] == {}
 
 
-# --- AR helper functions (v0.3.0 step 5, docs/v0.3.0-design.md §6) ---------
+# --- AR helper functions (v0.3.0 step 5) ---------
 
 
 def test_stick_fraction_matches_sigmoid_of_logit():
@@ -119,7 +119,7 @@ def test_ar_has_prev_false_only_at_slot_zero():
     assert has_prev.tolist() == [[False, True, True, True, True]]
 
 
-# --- _stage2_tf_prob (docs/v0.3.0-design.md §3.3, v0.3.0 step 7) -----------
+# --- _stage2_tf_prob (v0.3.0 step 7) -----------
 
 
 def test_stage2_tf_prob_always_is_constant_one():
@@ -569,7 +569,7 @@ def test_metrics_csv_columns_are_stage_prefixed():
 
 def test_wgan_stage_trainer_skips_generator_step_when_no_grad_this_batch():
     """Regression test: on a non-generator-step batch, if this stage's model
-    has no n_sec_head (n_sec defaults to stage 2, decision 1), g_loss is a
+    has no n_sec_head (n_sec defaults to stage 2), g_loss is a
     graph-less zero — .backward() must not be called on it."""
     cfg = _base_cfg()
     cfg["stage1_model"]["generator"] = "wgan"
@@ -672,11 +672,11 @@ def test_train_end_to_end_ar_attention_history_scheduled_teacher_forcing(
 
 
 def test_ar_wgan_onehot_grad_norm_instrumentation_populates_metrics():
-    """§11.4 differentiability validation-obligation instrumentation: the
+    """Differentiability validation-obligation instrumentation: the
     trunk-gradient-norm-by-slice columns must appear and actually fire for
     generator='wgan' + particle_type.target='onehot' under decoder=
-    'autoregressive' (added at v0.3.0 step 5 per the design doc's
-    instruction to accrue evidence during the architecture comparison)."""
+    'autoregressive' (added at v0.3.0 step 5 to accrue evidence during the
+    architecture comparison)."""
     cfg = _base_cfg()
     cfg["stage2_model"]["decoder"] = "autoregressive"
     cfg["stage2_model"]["particle_type"] = {"target": "onehot", "lambda": 1.0}