terminal: advance set lookup buckets directly

Carry the current RefCountedSet bucket through the probe loop instead of
recomputing hash plus probe length and narrowing it on every iteration.
The bucket remains in the set ID type and wraps with the existing table mask.

ReleaseFast timings are seven-run median user CPU times.

| Workload | Before | After | Change |
| --- | ---: | ---: | ---: |
| 128 styles, live lookup | 0.71s | 0.70s | -1.4% |
| 4,096 styles, live lookup | 0.89s | 0.85s | -4.5% |
| Alternating SGR stream | 1.11s | 1.11s | unchanged |

src/terminal/ref_counted_set.zig

@@ -593,8 +593,9 @@ pub fn RefCountedSet(
             const table = self.table.ptr(base);
             const items = self.items.ptr(base);
 
+            var p: Id = @truncate(hash);
+            p &= self.layout.table_mask;
             for (0..self.max_psl + 1) |i| {
-                const p: usize = @intCast((hash +% i) & self.layout.table_mask);
                 const id = table[p];
 
                 // Empty bucket, our item cannot have probed to
@@ -624,6 +625,8 @@ pub fn RefCountedSet(
                 {
                     return id;
                 }
+
+                p = (p +% 1) & self.layout.table_mask;
             }
 
             return null;