feat(c-miner): port miner pipeline to modular C implementation

miner.h

@@ -0,0 +1,30 @@
+#ifndef MINER_H
+#define MINER_H
+
+#include <stdatomic.h>
+#include <stdint.h>
+
+typedef void (*mine_status_cb)(long long attempts, double hashrate_hz, void *ctx);
+
+typedef struct {
+    int found;
+    uint32_t nonce;
+    double hashrate_hz;
+    long long attempts;
+    uint8_t header[80];
+    uint8_t digest[32];
+} MineResult;
+
+int mine_block(
+    uint8_t header_76[76],
+    const char *target_hex,
+    const char *nonce_mode,
+    uint32_t batch_size,
+    int timestamp_update_interval,
+    atomic_int *stop_flag,
+    mine_status_cb status_cb,
+    void *status_ctx,
+    MineResult *out
+);
+
+#endif