Select tandem AGC through the existing RX gain mode

[misko]

Selecting rx-gain-mode: tandem now acquires the existing kernel-backed paired AGC session. Selecting manual, slow_attack, or fast_attack releases tandem while preserving frame-associated metadata. PPlus.set_gain_mode() validates requests before mutation, verifies the first frame, restores the previous mode on failure, and stops RX if restoration fails.

The implementation adds strict metadata-v6 decoding, records requested modes/ownership/gaps in a backward-compatible V7 extension, pins the v0.49 host library, and preserves the metadata buffer when PyADI clears its reference on recoverable startup discards. It replaces the host-register-control approach in #27.

Requires the radio provider extension in misko/libiio#7. That extension is already deployed persistently on serial 104473b80a16000de6ff2000f8a6beca79 over the official v0.49 release; current DHCP address is 192.168.1.173. PPU's missing v0.49 diagnostic profile is addressed in misko/pluto-plus-utils#111.

Validation: 128 software tests; 10 receive-only hardware tests after reboot across 915 MHz, 2.41 GHz, and 5.75 GHz at two frame sizes; a final cleanup/rollback hardware smoke test; provider C tests with ASan/UBSan. Recorded 126 frames, 18 ownership epochs, 138 gain events, and no reported missing samples. Includes watchdog, process-exit, and disconnect/reconnect checks. Controlled RF amplitude/phase response and USB transport remain unqualified. Full deployment hashes, receipt reconciliation, and sanitized evidence are in docs/qualification/tandem-gain-mode-2026-09-12/.