feat: Replace the isDeleted flag by an int CLFlag (#55)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

packages/serverpod_offline_sync_server/lib/src/models/data/deleted.spy.yaml

@@ -1,20 +1,24 @@
-### CRDT tombstone table.
+### CRDT visibility generation table.
 ###
-### This table stores all tombstones for deleted data.
+### This table stores materialized monotone CLFlag metadata after a visibility
+### change event has been recorded.
 class: CrdtDataDeleted
 extends: BaseHlc
 table: crdt_data_tombstone
 database: all
 fields:
-  ### Row that was deleted.
+  ### Row whose visibility is tracked.
   row: CrdtDataRow?, relation(name=row_deleted, field=rowId, onDelete=Cascade)
   rowId: int
 
   ### The node that updated the field.
   node: CrdtNode?, relation
 
-  ### When the row was deleted.
-  isDeleted: bool
+  ### Monotone causal-length flag. Odd values are visible and even values are deleted.
+  clFlag: int
+
+  ### Why the row entered its current visibility generation.
+  reason: CrdtDataDeletedReason
 
 indexes:
   crdt_data_tombstone_row_idx:

packages/serverpod_offline_sync_server/lib/src/models/data/deleted_reason.spy.yaml

@@ -0,0 +1,10 @@
+### Why a CRDT row entered its current visibility generation.
+enum: CrdtDataDeletedReason
+serialized: byIndex
+values:
+  - userInsert
+  - userDelete
+  - userReinsert
+  - restrictRestore
+  - cascadeDelete
+  - uniqueLoser

packages/serverpod_offline_sync_server/lib/src/models/data/row.spy.yaml

@@ -22,7 +22,10 @@ fields:
   ### The node that inserted the row with the HLC timestamp.
   node: CrdtNode?, relation
 
-  ### The tombstone for this row.
+  ### The visibility CLFlag for this row.
+  ###
+  ### Only present after the first visibility change event has been recorded.
+  ### If null, the row is visible and has never been deleted/hidden.
   deleted: CrdtDataDeleted?, relation(name=row_deleted, optional)
 
   ### The fields for this row.

packages/serverpod_offline_sync_server/lib/src/models/merge/delete.spy.yaml

@@ -1,6 +1,9 @@
-### A remote tombstone update to merge.
+### A remote visibility CLFlag update to merge.
 class: CrdtMergeDelete
 extends: CrdtMergeChange
 fields:
-  ### Whether the row is deleted or restored.
-  isDeleted: bool
+  ### Monotone causal-length flag. Odd values are visible and even values are deleted.
+  clFlag: int
+
+  ### Why the row entered its current visibility generation.
+  reason: CrdtDataDeletedReason