Controllers are now pluggable by type with per-backend discovery:
- vxi11 backend (Tektronix AD007) via pyvisa-py, single-link discipline to
  avoid wedging the gateway; NetFinder stays the Prologix discovery.
- bench_discover aggregates all backends; discovery.scan_vxi11 (RPC/portmap).

Instruments live in a persistent pool (pool.toml + learned pool-state.json),
addressed by logical name and decoupled from the bridge:
- resolve() = pinned -> learned -> (opt-in) scan; auto-relocate OFF by default
  so normal reads never sweep a fragile gateway.
- named, savable sessions (sessions.json).
- MCP server reworked to 15 flat tools (pool_*, session_*, instrument_*).

Fluke 8840A driver:
- decode G0/G5/G6/G7 status (G7 '10nn' error codes); _status_query rejects a
  free-running measurement that races a status read.
- self_test: clear errors, fire Z0, wait silently ~30s, read G7 once
  (any command during Z0 is itself error 60). Verified against the front panel.

This unit has no AC option (F2/F5/F6 are no-ops).

Entire-Checkpoint: eab5a5a181e9