fix(1001): stop create_* from half-writing onto tombstoned IDs

Root cause: db::next_item_number scanned the visible CRDT index and the
content store but not the tombstone set, so it would hand out a numeric
ID whose CRDT entry had been tombstoned. crdt_state::write_item then
silently no-op'd the insert (tombstone-match guard) while the content
store and SQLite shadow happily accepted the row, producing a split-
brain half-write that was invisible to every CRDT-driven read path and
couldn't be cleaned up by delete_story / purge_story.

This change closes the loop:

- crdt_state::read::{is_tombstoned, tombstoned_ids} expose the
  tombstone set so callers outside crdt_state can consult it.

- db::next_item_number now scans tombstoned_ids() too. The allocator
  skips past tombstoned numeric IDs instead of treating their slots as
  free.

- write_item logs a WARN when it rejects a write for a tombstoned ID
  (was silent). The warn is a tripwire — if the allocator ever lets one
  slip through again we'll see it in the log.

- create_item_in_backlog adds two defence-in-depth checks:
    (a) before any write, reject if the allocator returned a
        tombstoned ID;
    (b) after the writes, call read_item to confirm the CRDT entry
        materialised. If not, roll back the content-store + shadow-DB
        rows via db::delete_item and return Err.

Regression tests cover the allocator skip, the is_tombstoned accessor,
and the create_item_in_backlog rollback path.

Out of scope for this commit:
- Recovery of the already-half-written items currently in the running
  pipeline (989, 1000, 1001) — Stage 2/3 of the plan, handled
  separately.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/src/crdt_state/read.rs

@@ -205,6 +205,37 @@ pub fn read_item(story_id: &str) -> Option<PipelineItemView> {
     extract_item_view(&state.crdt.doc.items[idx])
 }
 
+/// Return `true` if `story_id` has been tombstoned via `evict_item`.
+///
+/// Tombstoned IDs are invisible to `read_item` / `read_all_items` but `write_item`
+/// silently rejects writes to them.  Callers that allocate fresh IDs (notably
+/// `db::next_item_number`) must consult this to avoid handing out a tombstoned ID,
+/// which would cause a silent half-write split-brain (bug 1001).
+pub fn is_tombstoned(story_id: &str) -> bool {
+    let Some(state_mutex) = get_crdt() else {
+        return false;
+    };
+    let Ok(state) = state_mutex.lock() else {
+        return false;
+    };
+    state.tombstones.contains(story_id)
+}
+
+/// Snapshot of all tombstoned story IDs.
+///
+/// Used by `db::next_item_number` to compute the highest tombstoned numeric ID
+/// so the allocator can skip past it.  Returns an empty vec if the CRDT layer
+/// is not initialised.
+pub fn tombstoned_ids() -> Vec<String> {
+    let Some(state_mutex) = get_crdt() else {
+        return Vec::new();
+    };
+    let Ok(state) = state_mutex.lock() else {
+        return Vec::new();
+    };
+    state.tombstones.iter().cloned().collect()
+}
+
 /// Mark a story as deleted in the in-memory CRDT and persist a tombstone op.
 ///
 /// This is the eviction primitive for story 521 — it lets external callers