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_validate.py

@@ -11,8 +11,8 @@ _K_MAX = 5
 
 
 def _tiny_models(particle_type_cfg: dict | None = None):
-    """A fresh v0.3.0 pair: Stage1Model owns no n_sec_head (decision 1), so
-    n_sec always comes from Stage2OneShot."""
+    """A fresh v0.3.0 pair: Stage1Model owns no n_sec_head, so n_sec always
+    comes from Stage2OneShot."""
     s1 = Stage1Model(
         pdg_vocab=3,
         mat_vocab=2,