Phase E: Soft deletes (tombstones) for compliance-grade row deletion

Implements GDPR/CCPA-compatible row-level deletion without rewriting
the underlying Parquet. Tombstone markers live beside each dataset and
are applied at query time via a DataFusion view that excludes the
deleted row_key_values.

Schema (shared::types):
- Tombstone { dataset, row_key_column, row_key_value, deleted_at,
              actor, reason }
- All tombstones for a dataset must share one row_key_column —
  enforced at write so the query-time filter remains a single
  WHERE NOT IN (...) clause

Storage (catalogd::tombstones):
- Per-dataset AppendLog at _catalog/tombstones/{dataset}/
- flush_threshold=1 + explicit flush after every append — tombstones
  are high-value, low-frequency; durability on return is the contract
- Reuses storaged::append_log infra so compaction is already wired
  (POST .../tombstones/compact will work once we expose it)

Catalog (catalogd::registry):
- add_tombstone validates dataset exists + key column compatibility
- list_tombstones for the GET endpoint
- TombstoneStore exposed via Registry::tombstones() for queryd

HTTP (catalogd::service):
- POST /catalog/datasets/by-name/{name}/tombstone
    { row_key_column, row_key_values[], actor, reason }
  Returns rows_tombstoned count + per-value failure list (207 on
  partial success).
- GET same path lists active tombstones with full audit info.

Query layer (queryd::context):
- Snapshot tombstones-by-dataset before registering tables
- Tombstoned tables: raw goes to "__raw__{name}", public "{name}"
  becomes DataFusion view with
  SELECT * FROM "__raw__{name}" WHERE CAST(col AS VARCHAR) NOT IN (...)
- CAST AS VARCHAR handles both string and integer key columns
- Untombstoned tables register as before — zero overhead

End-to-end on candidates (100K rows):
- Pick CAND-000001/2/3 (Linda/Charles/Kimberly)
- POST tombstone -> rows_tombstoned: 3
- COUNT(*) drops 100000 -> 99997
- WHERE candidate_id IN (those 3) -> 0 rows
- candidates_safe view transitively excludes them
  (Linda+Denver: __raw__candidates=159, candidates_safe=158)
- Restart: COUNT still 99997, 3 tombstones reload from disk

Reversibility: tombstones are reversible deletes, not destruction.
Power users can still query "__raw__{name}" to see deleted rows.
Phase 13 access control is what stops a non-admin from accessing
__raw__* tables.

Limits / follow-up:
- Physical compaction not yet integrated — Phase 8's compact_files
  doesn't read tombstones during merge. Tombstoned rows are still
  on disk until that integration ships.
- Phase 9 journald event emission for tombstones not wired —
  tombstone records carry their own actor+reason+timestamp so the
  audit trail is intact, but cross-referencing with the mutation
  event log would help compliance reporting.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/PHASES.md

@@ -154,6 +154,15 @@
   - `crates/lance-bench` standalone pilot (Lance 4.0) avoids DataFusion/Arrow version conflict with main stack
   - 8-dimension benchmark on resumes_100k_v2 — see docs/ADR-019-vector-storage.md for scorecard
   - Decision: hybrid architecture. Parquet+HNSW stays primary (2.55× faster search at 100K in-RAM). Lance added as per-profile second backend for random access (112× faster), append (0.08s vs full rewrite), hot-swap (14× faster index builds), and scale past 5M RAM ceiling.
+- [x] Phase E: Soft deletes (tombstones) — 2026-04-16
+  - `shared::types::Tombstone` — { dataset, row_key_column, row_key_value, deleted_at, actor, reason }
+  - `catalogd::tombstones::TombstoneStore` per-dataset append-log at `_catalog/tombstones/{dataset}/`, flush_threshold=1 + explicit flush so every tombstone is durable on return (compliance requirement)
+  - All tombstones for a dataset must share the same `row_key_column` (validated at write — query filter is built as a single WHERE NOT IN clause)
+  - `Registry::add_tombstone / list_tombstones`
+  - Endpoint: `POST /catalog/datasets/by-name/{name}/tombstone` accepting `{row_key_column, row_key_values[], actor, reason}`; companion `GET` lists active tombstones
+  - `queryd::context::build_context` wraps tombstoned tables: raw goes to `__raw__{name}`, public name becomes a DataFusion view with `WHERE CAST(col AS VARCHAR) NOT IN (...)` filter
+  - End-to-end on candidates: tombstone 3 IDs, COUNT drops 100,000 → 99,997, specific WHERE returns empty, AiView candidates_safe transitively excludes them too, restart preserves all tombstones
+  - Limits / not in MVP: physical compaction (Phase 8 doesn't yet read tombstones during merge); journal integration (tombstones don't yet emit Phase 9 mutation events — covered by audit fields on the tombstone itself)
 - [x] Phase D: AI-safe views — 2026-04-16
   - `shared::types::AiView` — name, base_dataset, columns whitelist, optional row_filter, column_redactions
   - `shared::types::Redaction` — Null | Hash | Mask { keep_prefix, keep_suffix }