v7.16.0

The independent plugin release channel (RDR-197) and the plan-audit
termination contract. Engine identity is unchanged at
engine-service-v0.1.85 -- no engine cut rides this release.

Added

• Independent plugin release channel (RDR-197): anchored
  plugin-v{X.Y.Z}-{n} tags can now ship accumulated conexus//sn/
  plugin content without a client release. Channel primitives
  (scripts/plugin_channel.py: per-plugin ref invariants R and W with
  the cut-branch closer, blind-checkout sentinel, counter-less tag
  numbering, tag-anchored wheel-surface proof), the cut script
  (scripts/cut_plugin_release.py: main-readiness guard, atomic-split
  refusal with ledger bead attribution, deletion-safe allowlist
  import), a verify-only plugin-release.yml tag workflow that
  publishes nothing, per-plugin source.ref parity, per-plugin drift
  scoping, the release window accepted at all three sites, and a
  26-cell mixed-state truth table. Operator docs in AGENTS.md
  ("Independent plugin release channel") and docs/contributing.md
  ("Plugin channel cut"). This release carries the machinery to main,
  arming the channel; no cut has been performed yet.
• nx_plan_audit terminates (nexus-ll7zm): every finding is
  classified BLOCKS-PLANNING or DISCOVER-AT-IMPLEMENTATION; only
  the first can block; from round 3 (or a tighter caller-declared
  budget_rounds) the verdict is RESIDUALS-ONLY and planning is
  over. The verdict is computed from classified findings -- the
  model's own proposed verdict is not an input -- and a malformed
  findings payload synthesizes a blocking finding instead of reading
  as a clean audit. New round_number/budget_rounds parameters;
  nx_plan_audit_verdict structlog event. Enforcement in
  src/nexus/plans/audit_rounds.py; the four plugin guidance surfaces
  ship in this same release, closing the split delivery.
• Deliberate MCP wire shapes (nexus-r90ao): explicit
  structured_output declarations on all 46 MCP tools; search()
  ships real structuredContent.
• Expectations-ledger reconciliation (nexus-2v0v7):
  expectations_reconcile cross-checks outstanding background STARTs
  against the harness's own background_tasks ground truth (STRANDED /
  UNDECLARED_TASK classes), wired into the Stop hook as WARN-only.
• Named workflows pressure-test and dead-wire-census
  (nexus-hhqli).
• Plugin-eval corpus (nexus-7zup9): 15 skill-triggering cases under
  conexus/evals/, authored pending the org-gated claude plugin eval
  early access.

Fixed

• The T1 three-scopes record: the MCP server is NOT respawned on
  /clear//resume (nexus-ggvi0 falsification, proven from live
  logs); the handoff layer remains load-bearing and the re-proposal bar
  is documented in docs/architecture.md.