terminal: add compressed page representation

The standalone LZ4 codec had no representation for terminal page
ownership or metadata, so PageList integration would otherwise need to
reconstruct every Page field independently.

Add compress.Page, which embeds the complete terminal Page while
retaining its original virtual mapping and owns only an exact-sized
encoded block. Compression is kept only when the encoded state is
strictly smaller, and scratch output is capped at that profitability
boundary so a future PageList caller can borrow a standard pool item.

Extend the page-compression benchmark with a store mode that measures
the encoded copy, allocation, bounded retention, and eviction path.
Nothing uses compression from PageList yet; this remains isolated
groundwork.

src/terminal/main.zig

@@ -83,7 +83,7 @@ test {
 
     // Internals
     _ = @import("bitmap_allocator.zig");
-    _ = @import("compress/lz4.zig");
+    _ = @import("compress.zig");
     _ = @import("hash_map.zig");
     _ = @import("ref_counted_set.zig");
     _ = @import("size.zig");