content: content/sanitize — drop invalid blocks tree-wide (799/799)

Enforcement counterpart to validate: removes blocks failing validate's own
per-block id/field checks (reused via content/-block-issues, single-sourced)
so federated/imported input can render safely. Tree-wide; distinct from
normalize (empty vs invalid); keeps valid-shell sections, drops invalid ones.
New suite +12 tests (42 suites total).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/content/scoreboard.md

@@ -35,6 +35,7 @@ _Generated by `lib/content/conformance.sh`_
 | data | 25 | 0 | 25 |
 | wire | 11 | 0 | 11 |
 | validate | 32 | 0 | 32 |
+| sanitize | 12 | 0 | 12 |
 | store | 46 | 0 | 46 |
 | snapshot | 20 | 0 | 20 |
 | crdt | 34 | 0 | 34 |
@@ -45,4 +46,4 @@ _Generated by `lib/content/conformance.sh`_
 | md-import | 38 | 0 | 38 |
 | md-doc | 12 | 0 | 12 |
 | fed | 20 | 0 | 20 |
-| **Total** | **787** | **0** | **787** |
+| **Total** | **799** | **0** | **799** |