#1761 Add opt-in Galahad session storage (free non-commercial licence for 1 NVIDIA GPU)

说明

## Summary

Adds an optional Galahad backend for saving and restoring complete Strata conversation states through the existing `/slots/0?action=save|restore` API.

- Stores complete session images, including attention KV, recurrent state, checkpoints and draft state.
- Uses confirmed lookups, checkpoints after saves and store rehydration at startup.
- Adds configuration, CLI options, size limits and setup documentation in `docs/GALAHAD.md`.
- Keeps the existing file backend and default inference path unchanged. Galahad is imported only when explicitly enabled.

## Requirements and scope

Requires `galahad-kv==1.31.5`, Linux x86-64 and a licensed NVIDIA GPU.

Galahad offers a free non-commercial licence for one GPU, valid for 12 months and renewable:
https://github.com/corbenicai/galahad/wiki/Install#step-0-the-licence-every-install-needs-one

Save and restore are explicit client operations. This change does not add automatic prefix caching, Blaise retrieval or branching. It supports slot 0 only and cannot be combined with parallel requests or `slot_save_path`. Saved names are immutable.

## Validation

- Server and request-hardening regression suite passed: 288 tests.
- Final Galahad adapter suite passed: 11 tests, including authenticated HTTP save/restore, byte-exact mock round trips, error handling and temporary-file cleanup.
- Existing slot and security tests passed.
- Native library from the 1.31.5 wheel loaded and reported ABI 1.31.
- Initialisation without licence/key material correctly refused with `GLH-E17`.

A licensed native save/restart/restore cycle, GPU output parity and performance were not tested. No C++ or backend kernel files were changed.