MR-794 step 2: address PR #68 review — merge semantics, cardinality, residual

Five fixes from PR #68 review (Cursor Bugbot + Codex + Cubic):

* **scan_with_pending gains merge-shadow semantics** (Codex P1, Cubic P1#1):
  new `key_column: Option<&str>` parameter. When set, committed rows
  whose key value appears in any pending batch are excluded from the
  scan — making `scan_with_pending` correctly merge-semantic for chained
  updates instead of naively unioning. execute_update calls with
  Some("id"). Without this, a chained `update where age > 30` could
  match a row whose pending value already moved out of range.

* **Multi-delete on same table no longer trips ExpectedVersionMismatch**
  (Cursor Bugbot HIGH): open_table_for_mutation routes through
  reopen_for_mutation when staging.inline_committed has the table,
  using the post-inline-commit Lance version captured at record_inline
  time. The legacy open_for_mutation_on_branch fence (Lance HEAD ==
  manifest pinned) is correct cross-writer but wrong intra-query when
  deletes have already advanced HEAD on this table. Branch goes away
  when Lance ships two-phase delete (lance-format/lance#6658).

* **Cardinality validation consolidated** (Cursor LOW + Codex P2 +
  Cubic P1#2 + Cubic P2): new exec/staging::count_src_per_edge +
  enforce_cardinality_bounds shared by mutation and loader paths.
  Restores the missing min-cardinality check on the engine path.
  Loader Merge mode passes Some("id") to dedupe edges being updated
  by id (not double-count committed + pending). Loader Append mode
  and engine path pass None (ULID-generated ids never collide).

* **Dead count_rows_with_pending removed** (Cursor LOW): never called.

* **Misleading concat-helper comment fixed** (Cubic P3): claimed
  schema normalization the helper doesn't implement. Updated to match
  reality.

* **Documentation honesty** (Cubic P1#3): MR-794 narrows but doesn't
  eliminate the "Lance HEAD ahead of __manifest" drift class. Drift is
  unreachable for op-execution failures (the partial_failure test pins
  this), but a residual remains at the finalize→publisher boundary
  because Lance has no multi-dataset commit primitive: per-table
  commit_staged calls run sequentially before manifest commit. Updated
  docs/runs.md, docs/invariants.md §VI.25, docs/releases/v0.4.1.md to
  scope the claim precisely.

* **Failpoint test pinning the residual**: new
  mutation.post_finalize_pre_publisher failpoint + two tests in
  tests/failpoints.rs that confirm the documented residual behavior.
  Catches future regressions that widen the residual.

Test additions on tests/runs.rs:

* chained_updates_with_overlapping_predicate_respects_intermediate_value
* multi_statement_delete_on_same_node_table
* cascade_delete_node_then_explicit_delete_edge_on_same_table
* mutation_insert_edge_enforces_min_cardinality
* load_merge_mode_dedupes_edge_for_cardinality_count

113/113 engine integration tests pass (runs + end_to_end + consistency
+ staged_writes + validators). Failpoints feature build runs in CI.

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

docs/invariants.md

@@ -110,7 +110,7 @@ Specific defaults (timeout values, memory caps, TTL windows) are *configuration*
     *Status: aspirational — referential integrity at scale requires SIP-backed cross-table validation; not yet implemented. Cross-batch / cross-version uniqueness tracked in MR-714.*
 
 25. **Isolation: per-query snapshot; read-your-writes within and across queries in a session.** Each query reads from one consistent manifest version. Within a multi-statement mutation, the read subplan inside each write operator sees the writes from earlier statements. Across queries in a session, reads always resolve the latest manifest version — no reader pinning to older snapshots.
-    *Status: upheld for inserts/updates after MR-794 step 2+ — `MutationStaging`'s in-memory accumulator + `TableStore::scan_with_pending` (DataFusion `MemTable` union with the committed Lance scan) implements read-your-writes within a multi-statement mutation. Delete-touching mutations are limited to delete-only by parse-time D₂; closing the within-query RYW gap for deletes requires Lance's two-phase delete API (tracked: MR-793 / Lance-upstream).*
+    *Status: upheld for inserts/updates after MR-794 step 2+ — `MutationStaging`'s in-memory accumulator + `TableStore::scan_with_pending` (DataFusion `MemTable` union with the committed Lance scan, with merge-shadow semantics for chained updates) implements read-your-writes within a multi-statement mutation. Delete-touching mutations are limited to delete-only by parse-time D₂; closing the within-query RYW gap for deletes requires Lance's two-phase delete API (tracked: MR-793 / Lance-upstream lance-format/lance#6658). The "Lance HEAD ahead of `__manifest`" drift class is unreachable for op-execution failures (the partial-failure test pins this), but a narrowed residual remains at the finalize→publisher boundary because Lance has no multi-dataset commit primitive — see [docs/runs.md](runs.md) "Finalize → publisher residual".*
 
 26. **Durability before acknowledgement.** Commit returns only after the substrate has confirmed durable persistence. No "fast" or "fire-and-forget" durability levels.