Companion to rename_entity. Same transactional shape; different job:
rename mutates one row in place, merge consolidates two rows into
one.

merge_entities(source_id, target_id) -> dict
  - INSERT OR IGNORE re-points every (fact_id, source) to (fact_id,
    target); existing dual links collapse to a single target link
  - DELETE remaining (fact_id, source) rows
  - union source.aliases + source.name into target.aliases (case-
    insensitive dedup, target.name itself excluded)
  - DELETE source entity row
  - re-encode every formerly-source-linked fact via
    _compute_hrr_vector(commit=False) so its bundle reflects the new
    entity set
  - rebuild affected category banks
  - all in one transaction; rollback on any failure

Tests (9 new, 175/175 in full memory suite):
- re-point + source row deletion + facts_re_encoded count
- alias union with case-insensitive dedup
- dual-link collapse (fact linked to both source and target ends up
  with one target link, no duplicate)
- empty-source merge (zero facts) still merges aliases and deletes
  the source row, no re-encode
- ValueError on self-merge, KeyError on missing ids
- encoding_version refresh on re-encoded facts
- post-merge probe by source.name resolves via merged alias
- rollback atomicity on simulated _compute_hrr_vector failure

Live DB merges (entity_id=11, 4, 10 → 114):
  Before: 7 Apollo* rows, recall split between NousResearch#11 (95 facts) and
          NousResearch#114 (96 facts) for the same conceptual company
  After:  4 Apollo* rows. NousResearch#114 now holds 100 facts (95 source links
          collapsed to 4 unique-to-source via INSERT OR IGNORE).
          Probe recall consolidated and stronger:
            fact_id=431  +0.5291 → +0.6405
            fact_id=34   +0.5051 → +0.6196
            fact_id=71   +0.4030 → +0.4481
          Probes by every alias (canonical, AER, AEG, bare "Apollo")
          return identical top-3.

Doctor post-merge: status=warn (only the pre-existing 4 dangling
fact_id refs in fact_entities; merge introduced zero new orphans —
re-point happens before delete). Smoke probe('Apollo Energy Group')
raw_sim=+0.6448 in 20ms over 507 facts.

Still open after this commit:
- 4 orphan fact_entities rows (pre-existing fact deletions that
  didn't clean up the join table)
- entity_id=125 "Apollo Energy Resources LLC Sales" (1 fact) and
  entity_id=96 "Apollo_Investigation_Vault" (1 fact) — judgment
  calls on whether to merge into NousResearch#114 or leave separate
- record_feedback dual-write to trust_score, retrieval_count product
  question, memory_banks dead-write table — all deferred from
  earlier commits

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