feat(objectql): report dangling references left by isSystem writes (#4551)

#4441 enforces lookup referential integrity on the write path but exempts
isSystem writes, so seed replay / package install / boot provisioning can
still store a reference that points at nothing with nothing saying so.
PR #4511 recorded the residual; this closes it by reporting.

inspectDanglingReferences() walks non-readonly reference fields, probes each
stored id with the SAME private probe the write-path guard uses, and reports
object/record/field/target/id. Reports; never rewrites. Unprobeable targets
count into undetermined and unreadable objects into skipped, so an outage can
never read as a clean bill of health. Bounded per object and per pass, RBAC
link tables first; hitting a cap sets truncated.

Rides the existing ADR-0057 lifecycle clock (hourly, first run past boot)
rather than arming a second one. LifecycleService.sweep() is unchanged.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017gEHJN2NFpS9VMeURvakgD

Author: claude

.changeset/dangling-reference-sweep.md

@@ -0,0 +1,51 @@
+---
+"@objectstack/objectql": minor
+---
+
+feat(objectql): report dangling lookup references left behind by `isSystem` writes (#4551)
+
+#4441 made the write path refuse a `lookup` id that exists in no row of the
+object it references, but deliberately exempted `isSystem` writes: seed replay,
+package install and boot-time provisioning legitimately write rows in an order
+that is self-consistent only once the batch completes, and failing them closed
+would turn an ordering detail into a boot failure. The exemption is correct, and
+it left a real residual — **the platform itself could still store a reference
+pointing at nothing, and nothing said so.** PR #4511 recorded that residual
+rather than silently accepting it; this closes it, by reporting.
+
+`ObjectQL.inspectDanglingReferences()` walks the non-`readonly` reference fields
+of every registered object and names each stored id that resolves to no row:
+object, record id, field, target object, the id itself, and the holding row's
+tenant. It **reports and never rewrites** — the rows genuinely exist and were
+genuinely written, so nulling a link would make the stored data disagree with
+what happened, and whether the repair is to create the missing target, re-run
+the seed or delete the holder is a judgement the platform cannot make.
+
+Three properties are worth knowing before reading a report:
+
+- **Unknown is not healthy.** The probe is the same one the write path enforces
+  with. It answers "missing" only when it RAN and found nothing; an unregistered
+  target, an absent driver or a probe that throws is counted into
+  `undetermined`, and an unreadable object is named in `skipped`. A datasource
+  outage can therefore never read as a clean bill of health.
+- **`readonly` references are skipped**, as on the write path: the value there
+  was minted by the platform, and at least one is a sentinel by design
+  (`sys_metadata_history.recorded_by` stores `actor ?? 'system'` in a
+  `lookup('sys_user')`).
+- **Bounded, RBAC-first.** The RBAC link tables are visited first — a dangling
+  row there is a security-surface record that resolves to nothing, and the
+  audience-anchor gate has to resolve that very target to evaluate the grant.
+  Per-object and total row caps bound one pass; hitting one sets `truncated`, so
+  a report that stopped early cannot read as "everything was checked".
+
+It rides the existing ADR-0057 lifecycle clock (hourly, first run delayed past
+boot — exactly when seed-written references become checkable) rather than arming
+a second one, so the finding surfaces without an operator knowing to go looking.
+It runs *after* the sweep and is isolated from it in both directions.
+`LifecycleService.sweep()` is unchanged: tooling that calls it directly still
+gets policy enforcement and nothing else.
+
+New exports from `@objectstack/objectql`: `DanglingReference`,
+`DanglingReferenceReport`, `REFERENCE_SCAN_PRIORITY_OBJECTS`,
+`DANGLING_SCAN_ROWS_PER_OBJECT`, `DANGLING_SCAN_MAX_ROWS`. Nothing was removed
+or renamed, and #4441's enforcement is untouched.