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/warm_setup_cache.py

@@ -31,8 +31,8 @@ def run_warm_setup_cache(
     select the normalizer cache entry
     (`giant.data.setup_cache.normalizer_key`) — pass the same values a later
     `giant train` invocation will use so it hits this warmed entry. The two
-    conditioning axes are independent (docs/v0.3.0-design.md §3.1) and may
-    differ. `router_enabled`/`router_type`/`n_experts` only matter for
+    conditioning axes are independent and may differ.
+    `router_enabled`/`router_type`/`n_experts` only matter for
     `router_type == "process"` (warms that `n_experts`'s process map); the
     energy-router quantile summary is always collected regardless, so a
     later `--router-type energy` run never needs to rescan just to seed