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>

giant/analysis/render.py

@@ -51,8 +51,8 @@ def _figure_params_v2(mc: dict, run_meta: dict) -> dict:
     """`_figure_params` for a new-shape (nested) `model_config` — has a
     `stage1_model` key. Reports stage 1's architecture (the headline
     generator); stage 2's generator is only added (`mode_s2`) when it
-    differs from stage 1's, since a mixed run (docs/v0.3.0-design.md's
-    `stage1=flow` + `stage2=wgan` case) is the interesting exception, not
+    differs from stage 1's, since a mixed run (the `stage1=flow` +
+    `stage2=wgan` case) is the interesting exception, not
     the common case."""
     s1 = mc["stage1_model"]
     s2 = mc.get("stage2_model") or {}