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>

docs/testing.md

@@ -206,7 +206,8 @@ Concretely, the upgrade tier:
    canonical, pinned-tag deploy) → else the target-branch `main` tip (chaos deploy of the branch
    HEAD — the real predecessor the PR merges onto) → else the upgrade tier is skipped. An optional
    `tests/<recipe>/previous/` supplies version-specific repair to the base ONLY (stripped before the
-   head redeploy). `UPGRADE_BASE_VERSION` may still pin an explicit tag override.
+   head redeploy). (The old explicit `UPGRADE_BASE_VERSION` pin was removed in phase canon §2.G — the
+   dynamic last-green/step-back resolution makes it redundant.)
 2. orchestrator captures `head_ref` (preferring `$REF` — the PR head sha; falls back to the recipe
    checkout HEAD for non-PR `!testme`).
 3. on the upgrade tier: re-checkout the recipe to `head_ref` (the prev-tag base deploy reset the