benchmark: add dense style replacement workload

Add a replacement mode that keeps all but one requested style live
while repeatedly adding and releasing unique styles. This isolates
dense backward-shift deletion and ID reuse without triggering the
all-dead bulk reset.

Generate replacement styles on demand so long runs continue to
exercise misses instead of cycling through the resident working set.

src/benchmark/StyleSet.zig

@@ -18,6 +18,7 @@ opts: Options,
 page: terminal.Page,
 values: []style.Style,
 ids: []style.Id,
+cursor: u24 = 0x100000,
 
 pub const Options = struct {
     /// Number of distinct styles in the working set.
@@ -37,13 +38,29 @@ pub const Mode = enum {
     /// Rebuild a working set whose previous entries are all dead, then clear
     /// all of its references again.
     churn,
+
+    /// Keep the set nearly full while repeatedly adding and releasing unique
+    /// styles. This exercises dense deletion without the all-dead reset.
+    replace,
 };
 
+fn styleFor(n: u24) style.Style {
+    return .{ .fg_color = .{ .rgb = .{
+        .r = @truncate(n),
+        .g = @truncate(n >> 8),
+        .b = @truncate(n >> 16),
+    } } };
+}
+
 pub fn create(alloc: Allocator, opts: Options) !*StyleSet {
     if (opts.entries == 0 or opts.entries > style.Set.max_count) {
         log.err("entries must be between 1 and {}", .{style.Set.max_count});
         return error.InvalidEntries;
     }
+    if (opts.mode == .replace and opts.entries < 2) {
+        log.err("replace mode requires at least 2 entries", .{});
+        return error.InvalidEntries;
+    }
 
     const ptr = try alloc.create(StyleSet);
     errdefer alloc.destroy(ptr);
@@ -62,20 +79,28 @@ pub fn create(alloc: Allocator, opts: Options) !*StyleSet {
     errdefer alloc.free(ids);
 
     for (values, 0..) |*value, i| {
-        const n: u24 = @intCast(i + 1);
-        value.* = .{ .fg_color = .{ .rgb = .{
-            .r = @truncate(n),
-            .g = @truncate(n >> 8),
-            .b = @truncate(n >> 16),
-        } } };
+        value.* = styleFor(@intCast(i + 1));
     }
 
-    for (values, ids) |value, *id| {
-        id.* = try page.styles.add(page.memory, value);
-    }
-
-    if (opts.mode == .churn) {
-        for (ids) |id| page.styles.release(page.memory, id);
+    switch (opts.mode) {
+        .lookup, .churn => {
+            for (values, ids) |value, *id| {
+                id.* = try page.styles.add(page.memory, value);
+            }
+            if (opts.mode == .churn) {
+                for (ids) |id| page.styles.release(page.memory, id);
+            }
+        },
+        .replace => {
+            // Leave one slot for the unique style added by stepReplace while
+            // keeping the table dense enough to exercise deletion backshifts.
+            for (
+                values[0 .. values.len - 1],
+                ids[0 .. ids.len - 1],
+            ) |value, *id| {
+                id.* = try page.styles.add(page.memory, value);
+            }
+        },
     }
 
     ptr.* = .{
@@ -99,10 +124,27 @@ pub fn benchmark(self: *StyleSet) Benchmark {
         .stepFn = switch (self.opts.mode) {
             .lookup => stepLookup,
             .churn => stepChurn,
+            .replace => stepReplace,
         },
     });
 }
 
+fn stepReplace(ptr: *anyopaque) Benchmark.Error!void {
+    const self: *StyleSet = @ptrCast(@alignCast(ptr));
+
+    for (0..self.opts.loops) |_| {
+        for (self.values) |_| {
+            const id = self.page.styles.add(
+                self.page.memory,
+                styleFor(self.cursor),
+            ) catch return error.BenchmarkFailed;
+            std.mem.doNotOptimizeAway(id);
+            self.page.styles.release(self.page.memory, id);
+            self.cursor +%= 1;
+        }
+    }
+}
+
 fn stepLookup(ptr: *anyopaque) Benchmark.Error!void {
     const self: *StyleSet = @ptrCast(@alignCast(ptr));
 
@@ -131,7 +173,7 @@ fn stepChurn(ptr: *anyopaque) Benchmark.Error!void {
 test StyleSet {
     const alloc = std.testing.allocator;
 
-    inline for (.{ Mode.lookup, Mode.churn }) |mode| {
+    inline for (.{ Mode.lookup, Mode.churn, Mode.replace }) |mode| {
         const impl = try StyleSet.create(alloc, .{
             .entries = 64,
             .mode = mode,