feat(canon): §2.G strip UPGRADE_BASE_VERSION entirely (plausible verified dynamic-base green)

Gate satisfied — live: with the pin removed, plausible's upgrade tier resolves base 3.0.1+v2.0.0 via
the same-version step-back (canonical 3.1.0 == head 3.1.0 → newest-older = 3.0.1, NOT the broken
3.0.0) and passes install+upgrade green (level 5/5). The pin is redundant, so removed everywhere:
- meta.py KEYS entry (RecipeMeta field auto-drops; 15→14 keys).
- run_recipe_ci.resolve_upgrade_base override branch + docstrings.
- tests/unit/test_meta.py (count 15→14, dropped None-assert), test_upgrade_base.py (override test).
- docs/recipe-customization.md (regenerated table + mentions), docs/testing.md.
- tests/plausible/recipe_meta.py (pin removed), tests/bluesky-pds (re-enable note → dynamic base).
294 unit tests pass; lint clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/unit/test_meta.py

@@ -58,7 +58,6 @@ def test_missing_meta_yields_spec_baseline(tmp_path):
     assert meta.BACKUP_CAPABLE is None  # None = auto-detect (tri-state, not False)
     assert meta.EXPECTED_NA is None
     assert meta.READY_PROBE is None
-    assert meta.UPGRADE_BASE_VERSION is None
     assert meta.BACKUP_VERIFY is None
     assert meta.UPGRADE_EXTRA_ENV is None
     assert meta.EXTRA_ENV == {}
@@ -74,9 +73,9 @@ def test_registry_field_set_matches_dataclass():
     import dataclasses
 
     assert [f.name for f in dataclasses.fields(RecipeMeta)] == [k.name for k in KEYS]
-    # the 15 final keys, no more (the 3 P2-deleted legacy keys are gone from the registry,
-    # so any recipe_meta still setting them hard-fails the typo gate)
-    assert len(KEYS) == 15
+    # 14 final keys (UPGRADE_BASE_VERSION removed in phase canon §2.G; the 3 P2-deleted legacy keys
+    # are gone too — any recipe_meta still setting a removed key hard-fails the typo gate)
+    assert len(KEYS) == 14
     assert not [k for k in KEYS if k.deprecated]
     for gone in ("CHAOS_BASE_DEPLOY", "OIDC_AT_INSTALL", "SKIP_GENERIC"):
         assert gone not in {k.name for k in KEYS}