feat(torrents): add encryption policy

TORRENT_FEATURES.md

@@ -37,6 +37,10 @@ belong in the download UI. The Aria2 reference is the [1.37.0 manual](https://ar
 - Optional `bt-prioritize-piece` preview policy for the head, tail, or both
   ends of every selected file. The constrained policy is validated, persisted,
   normalized, and reapplied when a Torrent starts or retries.
+- One validated Torrent encryption policy mapped to Aria2's
+  `bt-force-encryption`, `bt-require-crypto`, and `bt-min-crypto-level`:
+  disabled, required obfuscated handshake, or forced ARC4 payload encryption.
+  The policy is persisted and reapplied when a Torrent starts or retries.
 - Optional `bt-remove-unselected-file` cleanup after completion when a
   selected-file subset is configured. Firelink requires explicit confirmation,
   reserves the unselected paths against competing downloads, keeps those
@@ -44,9 +48,9 @@ belong in the download UI. The Aria2 reference is the [1.37.0 manual](https://ar
   after observing Aria2's completion cleanup (or on terminal failure,
   cancellation, or reconfiguration).
 - Deterministic local Aria2 smoke coverage for metadata resolution, selected
-  output, piece priority, pause/resume, ownership, cancellation/removal,
-  unavailable trackers, daemon failure, and `bt-stop-timeout` terminal behavior;
-  RPC-boundary coverage is separate.
+  output, piece priority, encryption policy, pause/resume, ownership,
+  cancellation/removal, unavailable trackers, daemon failure, and
+  `bt-stop-timeout` terminal behavior; RPC-boundary coverage is separate.
 
 ## Priority tiers for remaining work
 
@@ -61,10 +65,7 @@ No remaining Tier 0 items.
    per-file priority option. Firelink therefore does not pretend that
    `select-file` is file priority; this remains pending an engine capability or
    a safe product-level model.
-2. **Encryption policy** — expose `bt-force-encryption`,
-   `bt-require-crypto`, and `bt-min-crypto-level` as one validated policy so
-   users cannot accidentally select contradictory combinations.
-3. **Tracker timing controls** — expose tracker connect timeout, request
+2. **Tracker timing controls** — expose tracker connect timeout, request
    timeout, and interval only when their effect on battery/network behavior is
    explained and persisted.
 
@@ -79,5 +80,5 @@ No remaining Tier 0 items.
 
 The first implementation in this task was remote `.torrent` metadata intake;
 follow-up implementations add stall-timeout control, bounded peer diagnostics,
-persisted tracker exclusion, piece-preview priority, and safe unselected-file
-removal.
+persisted tracker exclusion, piece-preview priority, safe unselected-file
+removal, and the validated encryption policy.