terminal: add standalone LZ4 block codec

Scrollback compression needs a codec that can be used from libghostty-vt
without pulling in libc, and we need to measure it before integrating it
with terminal page ownership.

This adds an allocation-free raw LZ4 block codec in scalar Zig.  Callers
provide the input, output, and fixed-size scratch table. The decoder
uses an exact-size output contract so page metadata mismatches fail
cleanly. Compatibility vectors, boundary cases, random round trips, and
fuzz coverage exercise the block format.

Also adds a page-compression benchmark that operates on reusable raw
page corpora. Compression and decompression have separate modes with
setup outside the timed region, plus a ratio report and no-op baseline.
Nothing uses compression in the terminal yet; this is the isolated codec
and measurement groundwork.

src/benchmark/cli.zig

@@ -8,6 +8,7 @@ const cli = @import("../cli.zig");
 pub const Action = enum {
     @"codepoint-width",
     @"grapheme-break",
+    @"page-compression",
     @"screen-clone",
     @"terminal-parser",
     @"terminal-stream",
@@ -25,6 +26,7 @@ pub const Action = enum {
     pub fn Struct(comptime action: Action) type {
         return switch (action) {
             .@"screen-clone" => @import("ScreenClone.zig"),
+            .@"page-compression" => @import("PageCompression.zig"),
             .@"terminal-stream" => @import("TerminalStream.zig"),
             .@"codepoint-width" => @import("CodepointWidth.zig"),
             .@"grapheme-break" => @import("GraphemeBreak.zig"),