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>

scripts/check_migration_v02_v03.py

@@ -1,6 +1,5 @@
-"""Portal-machine follow-up for v0.3.0 step 2 (docs/v0.3.0-design.md §4.3):
-diff a real v0.2 checkpoint's outputs against the new `build_models` on the
-same input batch.
+"""Portal-machine follow-up for v0.3.0 step 2: diff a real v0.2 checkpoint's
+outputs against the new `build_models` on the same input batch.
 
 `tests/test_migration_v02_v03.py` already proves this bit-identical with
 synthetic random weights, but that test can't run where it matters (no
@@ -16,7 +15,7 @@ Usage (from the repo root, on a portal machine):
     uv run python scripts/check_migration_v02_v03.py /ceph/lbogner/.../best.pt --batch 32 --seed 1
 
 Run it once against a flow (or ddpm) checkpoint and once against a wgan
-checkpoint (design doc §4.3's "one flow checkpoint and one WGAN checkpoint").
+checkpoint ("one flow checkpoint and one WGAN checkpoint").
 A routed checkpoint (`model_config["router"]["enabled"]`) is only checked for
 successful construction — `giant.model.network.migrate_legacy_state_dict`
 doesn't yet remap routed (Expert-per-router) state dicts, so the
@@ -115,8 +114,7 @@ def main() -> int:
         print(
             "  routed checkpoint: migrate_legacy_state_dict only handles the "
             "monolithic trunk shape — verifying construction only, skipping "
-            "the bit-identical weight/output comparison. See "
-            "docs/v0.3.0-design.md §2.4's scope note."
+            "the bit-identical weight/output comparison."
         )
         print("PASS (construction only, routed checkpoint)")
         return 0