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/update.zig

@@ -21,7 +21,12 @@ pub fn apply(doc: *bson.Document, update: *const bson.Document) UpdateError!void
     doc.pairs = try pairs.toOwnedSlice(arena);
 }
 
-fn apply_operator(arena: std.mem.Allocator, pairs: *std.ArrayListUnmanaged(bson.Pair), op: []const u8, value: bson.Value) UpdateError!void {
+fn apply_operator(
+    arena: std.mem.Allocator,
+    pairs: *std.ArrayListUnmanaged(bson.Pair),
+    op: []const u8,
+    value: bson.Value,
+) UpdateError!void {
     if (std.mem.eql(u8, op, "$set")) {
         const ops = doc_pairs(value) orelse return error.InvalidUpdate;
         for (ops) |p| {
@@ -146,7 +151,11 @@ fn parse_index(seg: []const u8) ?usize {
 }
 
 /// Shallow-copy a slice into a growable list backed by `arena`.
-fn copy_to_list(comptime T: type, arena: std.mem.Allocator, items: []const T) UpdateError!std.ArrayListUnmanaged(T) {
+fn copy_to_list(
+    comptime T: type,
+    arena: std.mem.Allocator,
+    items: []const T,
+) UpdateError!std.ArrayListUnmanaged(T) {
     var out: std.ArrayListUnmanaged(T) = .empty;
     errdefer out.deinit(arena);
     try out.appendSlice(arena, items);
@@ -173,7 +182,12 @@ fn get_value(pairs: []const bson.Pair, segs: []const []const u8) ?bson.Value {
     };
 }
 
-fn set_path(arena: std.mem.Allocator, pairs: *std.ArrayListUnmanaged(bson.Pair), segs: []const []const u8, value: bson.Value) UpdateError!void {
+fn set_path(
+    arena: std.mem.Allocator,
+    pairs: *std.ArrayListUnmanaged(bson.Pair),
+    segs: []const []const u8,
+    value: bson.Value,
+) UpdateError!void {
     if (segs.len == 1) {
         if (find_pair(pairs.items, segs[0])) |idx| {
             pairs.items[idx].value = value;
@@ -237,7 +251,11 @@ fn set_path(arena: std.mem.Allocator, pairs: *std.ArrayListUnmanaged(bson.Pair),
     }
 }
 
-fn unset_path(arena: std.mem.Allocator, pairs: *std.ArrayListUnmanaged(bson.Pair), segs: []const []const u8) void {
+fn unset_path(
+    arena: std.mem.Allocator,
+    pairs: *std.ArrayListUnmanaged(bson.Pair),
+    segs: []const []const u8,
+) void {
     if (segs.len == 1) {
         if (find_pair(pairs.items, segs[0])) |idx| {
             _ = pairs.orderedRemove(idx);