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/fuzz_split.zig

@@ -113,7 +113,12 @@ fn run(seed: u64, ops: usize, max_len: usize) !void {
                 if (std.mem.eql(u8, got, d.id)) hit = true;
             }
             if (!hit) {
-                std.debug.print("seed {d} op {d}: id {s} (key len {d}) not found by descent\n", .{ seed, op, d.id, d.s.len });
+                std.debug.print("seed {d} op {d}: id {s} (key len {d}) not found by descent\n", .{
+                    seed,
+                    op,
+                    d.id,
+                    d.s.len,
+                });
                 return error.EntryUnreachable;
             }
         }