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/spill2.zig

@@ -48,7 +48,12 @@ pub fn main() !void {
         const d = try doc_of(gpa, &pairs);
         _ = try ix.add_doc(gpa, d, id, false);
     }
-    std.debug.print("count={d} leaves={d} depth={d} overflow={d}\n", .{ ix.count(), ix.leaf_count, ix.depth, ix.overflow.items.len });
+    std.debug.print("count={d} leaves={d} depth={d} overflow={d}\n", .{
+        ix.count(),
+        ix.leaf_count,
+        ix.depth,
+        ix.overflow.items.len,
+    });
     if (ix.count() != N) return error.Bad;
 
     // Spot-check exact lookups.