style: adopt TigerStyle across src/; add docs/TIGER_STYLE.md

Wrap signatures and long expressions to the 100-column limit and make every
file zig fmt clean. Semantics-preserving throughout: ignoring whitespace and
the trailing commas that wrapping introduces, every file here is byte-identical
to its predecessor, and the one apparent exception is a warning string split
with `++`, which concatenates at comptime to the same bytes.

src/index.zig and src/commands.zig are reformatted in the commits that follow,
because their reformat is interleaved with in-flight changes to them and
separating the two would need the reformat re-derived rather than moved.

src/wire.zig

@@ -256,7 +256,13 @@ pub const Reply = struct {
     }
 
     /// Serialize this reply as a full OP_MSG message.
-    pub fn build(self: *Reply, gpa: std.mem.Allocator, request_id: u32, response_to: u32, out: *std.ArrayListUnmanaged(u8)) !void {
+    pub fn build(
+        self: *Reply,
+        gpa: std.mem.Allocator,
+        request_id: u32,
+        response_to: u32,
+        out: *std.ArrayListUnmanaged(u8),
+    ) !void {
         try write_message(gpa, request_id, response_to, 0, self.pairs.items, out);
     }
 };