fix(cold-eval): sample external inputs

src/signal_engine/cold_eval.cpp

@@ -30,10 +30,23 @@ void SignalEngine::init_defaults(Sample bpm, int beats_per_bar,
     GraphBuilder::init_symbols();
     state = EngineState{};
     session_generation = 0;
+    memset(cold_hw_inputs, 0, sizeof(cold_hw_inputs));
     reset_session_storage(bpm, beats_per_bar, bars_per_phrase,
                           phrases_per_section, false);
 }
 
+void SignalEngine::snapshot_cold_hw_inputs(const Sample* values,
+                                           uint16_t count) {
+    if (!values) count = 0;
+    if (count > MAX_HW_INPUT_CHANNELS) count = MAX_HW_INPUT_CHANNELS;
+    if (count > 0)
+        memcpy(cold_hw_inputs, values, count * sizeof(Sample));
+    if (count < MAX_HW_INPUT_CHANNELS) {
+        memset(cold_hw_inputs + count, 0,
+               (MAX_HW_INPUT_CHANNELS - count) * sizeof(Sample));
+    }
+}
+
 void SignalEngine::reset_session_storage(Sample bpm, int beats_per_bar,
                                          int bars_per_phrase,
                                          int phrases_per_section,
@@ -807,7 +820,6 @@ static EvalResult do_set(TokenStream& ts, SignalEngine& engine,
 
             Sample cell_vals[MAX_CELLS];
             engine.cells.snapshot_values(cell_vals, MAX_CELLS);
-            Sample hw_inputs[32] = {};
             Sample workspace[MAX_TOTAL_NODES] = {};
             Sample outputs[MAX_OUTPUTS] = {};
 
@@ -815,7 +827,7 @@ static EvalResult do_set(TokenStream& ts, SignalEngine& engine,
             ctx.t = engine.state.current_time;
             ctx.dt = engine.state.current_dt;
             ctx.cell_values = cell_vals;
-            ctx.hw_inputs = hw_inputs;
+            ctx.hw_inputs = engine.cold_hw_inputs;
             ctx.data_pool = engine.cells.data_pool;
             ctx.data_offsets = engine.cells.data_offsets;
             ctx.data_lengths = engine.cells.data_lengths;
@@ -2864,7 +2876,6 @@ EvalResult eval_expression(const char* source, uint32_t length,
     engine.cells.snapshot_values(cell_values, MAX_CELLS);
 
     // Execute one sample
-    Sample hw_inputs[32] = {};
     Sample outputs[MAX_OUTPUTS] = {};
     Sample workspace[MAX_TOTAL_NODES] = {};
 
@@ -2872,7 +2883,7 @@ EvalResult eval_expression(const char* source, uint32_t length,
     ctx.t = engine.state.current_time;
     ctx.dt = engine.state.current_dt;
     ctx.cell_values = cell_values;
-    ctx.hw_inputs = hw_inputs;
+    ctx.hw_inputs = engine.cold_hw_inputs;
     ctx.data_pool = engine.cells.data_pool;
     ctx.data_offsets = engine.cells.data_offsets;
     ctx.data_lengths = engine.cells.data_lengths;

src/signal_engine/cold_eval.h

@@ -161,6 +161,12 @@ struct SignalEngine {
     NodePool scratch_pool;
     char eval_text_buf[512] = {};
 
+    // Latest host-owned external-input snapshot used only by top-level cold
+    // expression evaluation. Hot graphs receive their input pointer through
+    // ExecutionContext on every tick. Keeping a copy here gives queries the
+    // same last-known values without retaining a host-lifetime pointer.
+    Sample cold_hw_inputs[MAX_HW_INPUT_CHANNELS] = {};
+
 #if USEQ_HAS_SYNTH_ENGINE
     // ── Host synth compiler domain (synth-nodes.md) ─────────────────────
     // Published synth artefacts: identity-keyed declarations + control
@@ -197,6 +203,8 @@ struct SignalEngine {
                                int bars_per_phrase = 4,
                                int phrases_per_section = 4,
                                bool publish_session_clear = true);
+
+    void snapshot_cold_hw_inputs(const Sample* values, uint16_t count);
 };
 
 // ── Cold-Path Evaluation ────────────────────────────────────────────────────

test/signal_engine/test_ext_registry.cpp

@@ -192,6 +192,29 @@ TEST_CASE("external inputs resolve as graph leaves", "[ext_registry]")
         REQUIRE(h.tick("a1", 1.5) == Approx(0.4));
     }
 
+    SECTION("cold expressions read the latest external-input snapshot")
+    {
+        REQUIRE(register_external_input(
+            {"nn/out", 4, 4, 0.5f, "[0,1]"}));
+        h.hw_inputs[4] = 0.1;
+        h.hw_inputs[5] = 0.2;
+        h.hw_inputs[6] = 0.3;
+        h.hw_inputs[7] = 0.4;
+        h.engine.snapshot_cold_hw_inputs(h.hw_inputs, 32);
+
+        EvalResult selected = h.eval("(nn/out 4)");
+        REQUIRE(selected.kind == EvalResult::Number);
+        REQUIRE(selected.number == Approx(0.4));
+
+        EvalResult composed = h.eval("(* 2 (nn/out 2))");
+        REQUIRE(composed.kind == EvalResult::Number);
+        REQUIRE(composed.number == Approx(0.4));
+
+        EvalResult invalid = h.eval("(nn/out 9)");
+        REQUIRE(invalid.kind == EvalResult::Number);
+        REQUIRE(invalid.number == Approx(0.5));
+    }
+
     SECTION("invalid multi-channel selectors return the declared neutral")
     {
         REQUIRE(register_external_input(