v7.20.0

Five instruments that reported absence when the truth was failure. Each said
"nothing here" — no MinerU, no search results, no stored row, no existing
document, no generation layout — while the real answer was that something had
broken, or that the question had been asked in a place it could not be
answered. nx self install also gains the route that makes the side-by-side
generation layout reachable from a packaged install at all.

Added

• nx self install converges a legacy uv-tool box (nexus-gu9zo). It now
  distinguishes three sites rather than two: a generation (build the next one),
  a legacy uv tool install conexus layout (CONVERGE it), and a genuine dev
  checkout (refuse, naming scripts/reinstall-tool.sh). Before this, the
  command refused everywhere that was not already a generation, so it could
  upgrade a generation box and never create the first one — and it told
  packaged users to run a repo script they have no copy of. Measured on a fresh
  uv tool install conexus of 7.19.0: uv-owned symlinks, no gen-*, no
  current, no <tools> directory at all. No packaged install of any age had
  the layout; every generation box in existence was checkout-driven, and
  commit 047dd80's migration had no caller anywhere in the tree. The converge
  never uninstalls: the legacy tree is registered as a pseudo-generation and
  reaped by a later, separate pass once nothing holds it, and extras bridge
  across from the legacy uv-receipt.toml — the only path by which [local]
  survives the move.

Fixed

• doctor's MinerU check asked "is it up", not "will it be there"
  (nexus-far1c). The check encoded the pre-nexus-1qdb9 model where MinerU was
  a long-lived server, so a correctly-idle on-demand service read as a hard
  failure, and its failure text asserted a fallback the module owning that
  fallback prevents. It now asks whether the server can come up when needed;
  reachability is informational, and an unprovisioned box still emits no row.
• A threshold drop is not an empty collection (nexus-1obui). search's
  zero-hit branch returned the nexus-uro6c diagnostic only on the prose path,
  so every programmatic caller — nx_tidy, the plan-runner step contract,
  anything reading ids/distances — could not tell "the threshold dropped
  every candidate" from "the collection is empty on this topic". The
  structured=True payload now carries it additively, and _tidy_prefetch
  names all six failure modes instead of collapsing them to "nothing to
  consolidate", so a T3 outage mid-tidy is no longer indistinguishable from a
  clean empty topic.
• memory_put reads the row back before claiming it stored (nexus-zra63).
  A returned row id proves the call returned, not that a row exists: a store
  that silently no-ops hands back the same shape as one that wrote. The write
  is now verified by re-reading and comparing, with three outcomes — verified,
  failed, and Stored (UNVERIFIED) when the read path itself is unavailable.
  That third state is the point: a check that reports success when persistence
  is unavailable is the same defect one level up.
• A curator-side reindex no longer forks a repo-owner document
  (nexus-tqudo). nx index repo registers under a repo owner while the
  doc_indexer family resolves a curator owner, and by_file_path is
  owner-scoped — so the curator-side lookup could never see the repo-owner row,
  structurally, and minted a second catalog Document for one physical file.
  --force was never causally involved. The lookup now checks the file's repo
  owner before minting.
• One rule resolves uv's tool directory (nexus-orhp5). Four separate rules
  answered "where is uv's tool dir" and disagreed — a substring test, a
  UV_TOOL_DIR-only reader, two shell-outs to uv tool dir, and a hardcoded
  ~/.local/share/uv/tools/.... install_layout.uv_tool_root() is now the
  single Python answer, implementing uv's own precedence (UV_TOOL_DIR >
  $XDG_DATA_HOME/uv/tools > ~/.local/share/uv/tools, measured against uv
  0.8.0), and the substring test became a real containment check. This stopped
  being cosmetic when the weakest of those rules began routing install
  convergence.

Documentation

• nx self install's three sites, the converge route and its live-holder
  guarantee are documented in docs/cli-reference.md; README's upgrade section
  no longer tells uv-tool users to run uv tool upgrade conexus instead, which
  would have kept them on the legacy layout permanently.