terminal/snapshot: skip remap tables for pages without styles

Decoding a page allocated and zeroed two full remap tables (a 128 KB
entries array plus an 8 KB seen bitmap each for styles and hyperlinks)
even when the page declared no table entries at all, which is every page
of plain scrollback. Empty tables now use a shared `.empty` remap that
allocates nothing; `get` reads it as all-unmapped through a length check.
Pages that do declare entries are unchanged.

(Leaving the entries array unzeroed behind a seen-bitmap-gated `get` was
also tried and measured no better than the plain memset, so the table
keeps its simple zero-means-unmapped representation.)

Benchmarks ("prev" is the parent commit):

| wasm      | encode prev | encode   | decode prev | decode   |
|-----------|------------:|---------:|------------:|---------:|
| ascii     |     2.06 ms |  2.13 ms |     2.70 ms |  2.54 ms |
| styled    |     2.41 ms |  2.29 ms |     6.56 ms |  6.75 ms |
| truecolor |     5.02 ms |  4.95 ms |    11.86 ms | 12.05 ms |
| cjk       |     6.03 ms |  6.23 ms |    11.60 ms | 11.39 ms |
| grapheme  |     8.36 ms |  8.72 ms |    11.39 ms | 11.27 ms |

| native | mode   | prev    | this    |
|--------|--------|--------:|--------:|
| ascii  | encode | 25.6 ms | 24.4 ms |
| ascii  | decode | 48.1 ms | 45.1 ms |
| utf8   | encode | 41.7 ms | 41.9 ms |
| utf8   | decode | 58.5 ms | 58.6 ms |

src/terminal/snapshot/grid.zig

@@ -1515,6 +1515,11 @@ fn Remap(comptime Id: type) type {
         /// semantics.
         seen: std.DynamicBitSetUnmanaged,
 
+        /// A remap with no entries at all: every lookup is unmapped. Use
+        /// this instead of `init` when the encoded table is empty so pages
+        /// without styles or hyperlinks allocate nothing.
+        pub const empty: Self = .{ .entries = &.{}, .seen = .{} };
+
         pub fn init(alloc: Allocator) Allocator.Error!Self {
             const entries = try alloc.alloc(Id, capacity);
             errdefer alloc.free(entries);
@@ -1527,25 +1532,29 @@ fn Remap(comptime Id: type) type {
         }
 
         pub fn deinit(self: *Self, alloc: Allocator) void {
-            alloc.free(self.entries);
-            self.seen.deinit(alloc);
+            if (self.entries.len != 0) {
+                alloc.free(self.entries);
+                self.seen.deinit(alloc);
+            }
             self.* = undefined;
         }
 
-        /// Record one encoded-to-native mapping.
+        /// Record one encoded-to-native mapping. Illegal on `empty`.
         pub fn put(self: *Self, encoded: Id, native: Id) void {
             assert(!self.seen.isSet(encoded));
             self.entries[encoded] = native;
             self.seen.set(encoded);
         }
 
-        /// Whether the encoded ID already has an entry, even a default one.
+        /// Whether the encoded ID already has an entry, even a default
+        /// one. Illegal on `empty`.
         pub fn contains(self: *const Self, encoded: Id) bool {
             return self.seen.isSet(encoded);
         }
 
         /// The native ID for an encoded ID, or zero when unmapped.
         pub inline fn get(self: *const Self, encoded: Id) Id {
+            if (self.entries.len == 0) return 0;
             return self.entries[encoded];
         }
     };

src/terminal/snapshot/page.zig

@@ -367,12 +367,19 @@ fn decodePayloadBody(
     page.pauseIntegrityChecks(true);
     defer page.pauseIntegrityChecks(false);
 
-    var style_remap = grid.StyleRemap.init(alloc) catch
-        return error.OutOfMemory;
+    // Pages without styles or hyperlinks, the common case for plain
+    // scrollback, skip the remap tables entirely: every encoded cell ID
+    // resolves to the default through the empty remap.
+    var style_remap: grid.StyleRemap = if (header.style_count > 0)
+        grid.StyleRemap.init(alloc) catch return error.OutOfMemory
+    else
+        .empty;
     defer style_remap.deinit(alloc);
 
-    var hyperlink_remap = grid.HyperlinkRemap.init(alloc) catch
-        return error.OutOfMemory;
+    var hyperlink_remap: grid.HyperlinkRemap = if (header.hyperlink_count > 0)
+        grid.HyperlinkRemap.init(alloc) catch return error.OutOfMemory
+    else
+        .empty;
     defer hyperlink_remap.deinit(alloc);
 
     // Styles