Implement De Bruijn-based top priority helper

Implement the helper function that uses a De Bruijn multiply-and-LUT
approach to compute the index of the least-significant set bit in O(1)
time complexity.

This helper is not yet wired into the scheduler logic; integration
will follow in a later commit. No functional change in this patch.

Author: vicLin8712

kernel/task.c

@@ -342,6 +342,18 @@ static const uint8_t debruijn_lut[32] = {
     0,  1,  28, 2,  29, 14, 24, 3, 30, 22, 20, 15, 25, 17, 4,  8,
     31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18, 6,  11, 5,  10, 9};
 
+/* O(1) priority selection optimized for RISC-V */
+static inline uint8_t find_highest_ready_priority(uint32_t bitmap)
+{
+    /* Isolate rightmost set bit (highest priority) */
+    uint32_t isolated = bitmap & (-bitmap);
+
+    /* De Bruijn multiplication for O(1) bit position finding */
+    uint32_t hash = (isolated * 0x077CB531U) >> 27;
+
+    return debruijn_lut[hash & 0x1F];
+}
+
 /* Weak aliases for context switching functions. */
 void dispatch(void);
 void yield(void);