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/dwarf.py

@@ -536,7 +536,7 @@ def warm_cache(
             "--material-conditioning",
             help="Must match the `giant train` run(s)' conditioning.material.type "
             "to warm for — independent of --particle-conditioning "
-            "(docs/v0.3.0-design.md §3.1: the two axes may differ)",
+            "(the two axes may differ)",
         ),
     ] = Conditioning.physical,
     router: Annotated[