fix(audio8_tts): Falcon-H1 0.1B logits parity and long-run stability

Three root causes made the Falcon-H1 slow-AR path diverge from the
transformers reference (first-frame argmax 3620 vs 2732, ASR round-trip
said the wrong word) and blew the SSM state up on long sequences:

1. conv1d kernel was flipped at load time, but ggml ssm_conv and the HF
   FalconH1 decode (nn.Conv1d prefill and the cached torch.sum path
   alike) use the same cross-correlation orientation with window[0] the
   oldest frame. Feed the GGUF kernel unflipped.
2. sx (conv window) and k_r/v (fresh K/V) were read back on the host
   without ggml_set_output, so gallocr reused their buffers and the
   conv/SSM/KV states were fed garbage every step. Pin exactly those
   three tensors per layer.
3. The host KV cache used the current sequence length as the per-head
   stride while appending only the new token, so from the second token
   on the append overwrote the previous head blocks and every head past
   the first read corrupted context. The resulting residual-stream
   garbage - not the recurrent scan itself - drove the SSM state to
   1e15..1e18 around token 180. append_falcon_kv_token now re-lays the
   cached tokens into the new stride before appending.

Verified against an f32 transformers 4.57.6 reference forced onto the
recurrent path: per-layer conv/SSM/KV states match within bf16 rounding
over the whole prompt, first-frame argmax = 2732, synthesized speech
round-trips through ASR verbatim, and a 605-position generation stays
bounded (max state ~1.1e3, no NaN) on both CPU and Metal backends.

Adds a regression test for the KV cache re-stride that fails against
the previous append logic at the second token.

Author: gqf2008

CMakeLists.txt

@@ -2089,6 +2089,17 @@ if (ENGINE_BUILD_TESTS)
         COMMAND safetensors_offsets_test
     )
 
+    add_engine_unittest(audio8_tts_falcon_kv_cache_test tests/unittests/test_audio8_tts_falcon_kv_cache.cpp)
+    target_include_directories(audio8_tts_falcon_kv_cache_test PRIVATE
+        ${CMAKE_CURRENT_SOURCE_DIR}/src/community_models/audio8_tts
+        ${CMAKE_CURRENT_SOURCE_DIR}/tests/unittests
+    )
+
+    add_test(
+        NAME audio8_tts_falcon_kv_cache_test
+        COMMAND audio8_tts_falcon_kv_cache_test
+    )
+
     add_engine_unittest(audio_chunking_test tests/unittests/test_audio_chunking.cpp)
 
     add_test(