salience: client-side pad expansion, drop AppendImage

Mirrors the vLLM-side rewrite. AppendImage is gone; images now
ride along on Generate via a parallel `images` list.

- Productionize `qwen3_image_token_count` (was test-only). Image
  leaf computes its IMAGE_PAD count eagerly at construction from
  height/width; `token_count` is no longer "0 until the server
  tells us."
- WireChunk shrinks to a single `Tokens(Vec<u32>)` variant — vision
  blocks live inline in the token stream.
- `wire_chunks` now returns `(Vec<WireChunk>, Vec<WireImage>)`.
  `WireImage` carries `pad_start` / `pad_end` (absolute positions
  in the full walk) alongside bytes + mime.
- `assemble_prompt` returns `(chunks, images, match_upto)`.
- `stream_session_mm` / `run_session_generate` take the parallel
  images list, filter to those past `match_upto`, and pass them
  in `GenerateRequest.images` as `pb::ImageAttachment` entries.
- Drop `SessionHandle::append_image`,
  `ContextState::commit_image_token_counts`,
  `StreamToken::ImageAppended`, the WireChunk::Image branch in
  `learn.rs`, and the now-empty `prompt_to_chunks` helper.
- Add 'v' toggle on the conscious-screen tree to render token-id
  vectors in place of text content (debug-aid: lets us see what
  the server actually has when output is suspicious).
- Comment out the subconscious-trigger spawn loop — Kent had this
  disabled before; it had crept back into running.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/mind/mod.rs

@@ -693,7 +693,7 @@ impl Mind {
             }
         });
 
-        let mut sub_handle: Option<tokio::task::JoinHandle<()>> = None;
+        let _sub_handle: Option<tokio::task::JoinHandle<()>> = None;
 
         // Start finetune scoring at startup (scores existing conversation)
         if !self.config.no_agents {
@@ -743,6 +743,7 @@ impl Mind {
                 _ = tokio::time::sleep(timeout), if !has_input => _dmn_expired = true,
             }
 
+            /*
             if !self.config.no_agents {
                 if sub_handle.as_ref().map_or(true, |h| h.is_finished()) {
                     let sub = self.subconscious.clone();
@@ -754,6 +755,7 @@ impl Mind {
                     }));
                 }
             }
+            */
 
             // Check for pending user input → push to agent context and start turn
             let pending = self.shared.lock().unwrap().take_pending_input();