v2.18.3 — Spec hygiene, via_table E2E, RRF text-leg test, fusion exception

v2.18.3 (2026-09-07)

Spec hygiene: updated specs 044 and 040; added missing E2E tests

• Spec 044 US7: Added Phase 10 documenting the kg_TXT calling-convention fix,
  kg_RRF_FUSE text-leg validation, and fusion.py exception propagation.
  7 E2E tests in tests/integration/test_kg_txt_e2e.py — all passing.
• Spec 040 T030: test_via_table_traversal E2E (M:M join-table path) implemented
  and passing; task marked [X] in spec 040 tasks.md.
• test_multi_modal_text_failure / test_multi_modal_no_results: Updated to
  assert RuntimeError is raised (not silently swallowed) when kg_TXT fails.
  This matches the correct behavior introduced in v2.18.2.