kv-cells: stop the sequence scan once all sequences are seen (#28011)

for_each_token_in tested all LLAMA_MAX_SEQ sequences for every used cell,
while a cell almost always belongs to one. The scan now stops once the
cell's own sequences have been seen. Same visit order, same callback
arguments, so behaviour is unchanged.

get_prev_tokens is the only caller, so this affects the n-gram path.

RTX PRO 6000, Qwen3.8-Flash-Next UD-Q4_K_XL, fa on, warm runs:

  55k context    generation 56.3 -> 74.3 t/s
  132k context   generation 33.6 -> 50.9 t/s

Prompt processing is unchanged, the scan is amortised over the ubatch
there. The gain follows the number of used cells, so it grows with
context and is invisible on short prompts.

src/llama-kv-cells.h

@@ -320,13 +320,14 @@ public:
             }
 
             const auto m = seq[i] & seqs;
-            if (m.none()) {
-                continue;
-            }
 
-            for (llama_seq_id s = 0; s < LLAMA_MAX_SEQ; ++s) {
+            // a cell carries a handful of sequences at most, out of LLAMA_MAX_SEQ
+            size_t left = m.count();
+
+            for (llama_seq_id s = 0; left > 0 && s < (llama_seq_id) LLAMA_MAX_SEQ; ++s) {
                 if (m.test(s)) {
                     f(s, pos[i], ext[i].tok);
+                    --left;
                 }
             }
         }