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_sample.py

@@ -1,6 +1,6 @@
 """Tests for giant/sample.py's v0.3.0 stage-model sampling — the AR loop
 (`sample_secondaries_ar`) and non-"physical" `particle_type.target` coverage
-for the one-shot samplers (docs/v0.3.0-design.md step 6)."""
+for the one-shot samplers."""
 
 import pytest
 import torch
@@ -38,7 +38,7 @@ def _cond(B: int, pdg: int = 3, mat: int = 2) -> tuple[torch.Tensor, torch.Tenso
 
 def _conditioning_for(target: str) -> str:
     # target="embedding" regresses against the conditioning's own embedding
-    # table (docs/v0.3.0-design.md §3.3) — only meaningful when the
+    # table — only meaningful when the
     # conditioning axis is itself "embedding".
     return "embedding" if target == "embedding" else "physical"
 
@@ -103,7 +103,7 @@ def _expected_type_dim(target: str, emb_dim: int) -> int:
     return PARTICLE_PHYS_DIM if target == "physical" else emb_dim
 
 
-# ── Stage-1 n_sec ownership (decision 1) ────────────────────────────────────
+# ── Stage-1 n_sec ownership ──────────────────────────────────────────────────
 
 
 def test_sample_flow_returns_none_n_sec_when_stage1_owns_no_head():