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/data/loader.py

@@ -176,7 +176,7 @@ def iter_file_chunks(
     """Yield one parquet row-group at a time so a large file never fully loads.
 
     `k_max` sets the padded width of the sec_*_list columns (should match
-    `stage2_model.k_max` — see docs/v0.3.0-design.md §8); defaults to the
+    `stage2_model.k_max`); defaults to the
     module constant for callers that don't care (e.g. Stage-1-only reads)."""
     pf = pq.ParquetFile(path)
     for i in range(pf.num_row_groups):
@@ -277,8 +277,7 @@ def _topn_plus_other_map(counts: dict, n_classes: int) -> tuple[dict, dict]:
 
     Returns `(class_map, other_members)` — `other_members` is `{key: count}`
     for every key bucketed into "other" (the empirical within-bucket
-    distribution, for `other_policy = "sample"` at rollout — see
-    docs/v0.3.0-design.md §8).
+    distribution, for `other_policy = "sample"` at rollout).
     """
     ranked = sorted(counts, key=lambda k: counts[k], reverse=True)
     keep = ranked[: max(n_classes - 1, 0)]
@@ -324,7 +323,7 @@ def build_topn_map_from_files(
     `_topn_plus_other_map`), generalized over the source column and key type.
 
     Used for the material axis (`column="material"`, `cast=str`, matching
-    `mat_map`'s key type) — see docs/v0.3.0-design.md §8. The PDG axis uses
+    `mat_map`'s key type). The PDG axis uses
     `build_pdg_topn_map_from_files` instead (it needs to pool two columns,
     which this single-column form can't express). Also records
     `other_members` (the empirical within-"other" distribution), needed
@@ -341,8 +340,8 @@ def build_pdg_topn_map_from_files(files: list[Path], n_classes: int) -> TopNMap:
     plays in this dataset: a step's own primary particle (`pdg` column) and
     an emitted secondary's species (`sec_pdg_list`, exploded) — shared by
     `conditioning.particle.type = "onehot"` and
-    `stage2_model.particle_type.target = "onehot"` (docs/v0.3.0-design.md
-    §8). Pooling both is what keeps a species that's common as a secondary
+    `stage2_model.particle_type.target = "onehot"`. Pooling both is what
+    keeps a species that's common as a secondary
     but rare as a primary (or vice versa) from being pushed into "other"
     just because one role's count alone looks small — the meeting's failure
     mode (zero photon secondaries, hallucinated antineutrinos) was