terminal/snapshot: gate page verification on slow runtime safety

PAGE decoding verified the complete native integrity of every decoded
page unconditionally, building per-cell reference maps that accounted
for roughly a fifth of decode time. The decoder normalizes every
semantic value while decoding, so a completed decode upholds page
invariants by construction and the verification only defends against
decoder bugs. Follow the native page policy instead: assertIntegrity
and friends run full verification only when slow runtime safety is
enabled, which keeps the check in debug and test builds where those
bugs are caught.

Benchmark deltas at this commit (terminal-snapshot, 1 MB corpora):

  ascii lines 1-70:  decode 15.5 -> 12.2 ms (encode unchanged)

src/terminal/snapshot/page.zig

@@ -93,6 +93,7 @@
 //! Rows and cells use the grid encoding documented in `grid.zig`.
 
 const std = @import("std");
+const build_options = @import("terminal_options");
 const Allocator = std.mem.Allocator;
 const test_fixture = @import("fixture.zig");
 const grid = @import("grid.zig");
@@ -227,7 +228,15 @@ pub const Decoder = struct {
             self.header,
         );
         try self.record_reader.finish();
-        try destination.verifyIntegrity(alloc);
+
+        // The decoder normalizes every semantic value, so a complete decode
+        // upholds native page invariants by construction. Verifying them
+        // again is a defense against decoder bugs and follows the native
+        // page policy: full integrity verification only when slow runtime
+        // safety is enabled.
+        if (comptime build_options.slow_runtime_safety) {
+            try destination.verifyIntegrity(alloc);
+        }
     }
 };