common : re-arm reasoning budget after DONE on new <think> (#22323)

DONE state absorbs all tokens including a new start tag, causing any think blocks after the first to run unbudgeted. Observed on unsloth/Qwen3.6-27B-GGUF which interleaves multiple <think> blocks per response.

Fixed by advancing start_matcher in DONE branch and re-arming to COUNTING with a fresh budget on match. Adds regression test (test-reasoning-budget: test 6).

tests/test-reasoning-budget.cpp

@@ -227,7 +227,30 @@ int main(void) {
             3);     // forcing continues through i=3
     }
 
-    printf("OK (5 tests passed)\n");
+    // Test 6: Multi-block thinking. First block ends naturally at i=2, second
+    // start tag at i=3 re-arms the budget, which then exhausts at i=5.
+    // Regression: before this fix, DONE absorbed all subsequent tokens and a
+    // second <think> block ran unbudgeted.
+    // Flow: i=0 accept(100)->COUNTING rem=2; i=1 accept(50)->rem=1;
+    //       i=2 accept(101)->end_matcher matches, DONE;
+    //       i=3 accept(100)->re-arm, COUNTING rem=2;
+    //       i=4 accept(60)->rem=1; i=5 accept(61)->rem=0->FORCING;
+    //       i=6 apply()->forces token[0]=102, accept(62)->force_pos=1, stay FORCING;
+    //       i=7 apply()->forces token[1]=101, accept(63)->force_pos=2->DONE.
+    {
+        const std::vector<llama_token> start = {100};
+        const std::vector<llama_token> end = {101};
+        const std::vector<llama_token> forced = {102, 101};
+        const std::vector<llama_token> sequence = {100, 50, 101, 100, 60, 61, 62, 63};
+
+        test_reasoning_budget("multi-block re-arms budget after DONE", sequence, start, end, forced,
+            2,      // budget of 2 tokens (per block)
+            REASONING_BUDGET_IDLE,
+            6,      // forcing starts at i=6 (after second block exhausts at i=5)
+            7);     // forcing continues through i=7
+    }
+
+    printf("OK (6 tests passed)\n");
 
     printf("Testing UTF-8 boundary detection... ");
     test_utf8_boundary_detection();