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.

README.md

@@ -5,6 +5,13 @@ Zig 0.16. Like SQLite, it stores everything in a single file; unlike SQLite,
 it speaks the MongoDB wire protocol, so real clients — `mongosh`, the Node.js
 driver, PyMongo — connect over TCP and just work.
 
+## Forward plan
+
+The direction from this MVP — a full-fledged embedded, tens-of-GB,
+maximally MongoDB-compatible database — its decision record, milestones
+and gates live in [PLAN.md](PLAN.md). Milestone 0 (mmap + WAL storage
+foundation) is next.
+
 ## Quick start
 
 ```sh
@@ -317,6 +324,9 @@ Several real bugs surfaced while benchmarking:
 
 ## Code style
 
-Zig 0.16 idioms (`std.Io` threaded through everything, unmanaged
-containers); user-declared functions use `snake_case` per this repo's house
-style.
+The project follows TigerBeetle's TigerStyle — see
+[`docs/TIGER_STYLE.md`](docs/TIGER_STYLE.md) (binding reference) and the
+"Code style" section of `AGENTS.md` for the project-specific rules and
+deliberate deviations. Highlights: `zig fmt` clean, 100-column hard limit,
+4-space indent, snake_case, functions under 70 lines, always-on
+assertions via `src/assert.zig`.