docs: add project readme

README.md

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+# gambit
+
+A chess engine written in Scala 3, built with [scala-cli](https://scala-cli.virtuslab.org/).
+
+## Overview
+
+`gambit` is a from-scratch alpha-beta chess engine. It uses a `0x88`
+mailbox board representation, Zobrist hashing for transposition detection,
+and a principal-variation search with a full set of modern pruning and
+move-ordering heuristics. It speaks the UCI protocol and can also read and
+write SAN, PGN and EPD.
+
+## Features
+
+- **Board** — `0x88` mailbox representation with incremental Zobrist hashing.
+- **Move generation** — pseudo-legal and legal generation for all piece
+  types, including castling, en passant and promotions, validated by a
+  `perft` node counter.
+- **Evaluation** — tapered midgame/endgame evaluation with piece-square
+  tables, pawn structure (passed, doubled, isolated), mobility, the bishop
+  pair, rook file bonuses and king safety, plus a static exchange
+  evaluation (SEE).
+- **Search** — iterative-deepening PVS with quiescence search, a
+  transposition table, null-move pruning, reverse futility and razoring,
+  late-move reductions, late-move pruning, killer moves and a history
+  heuristic, with aspiration windows at the root.
+- **Time management** — UCI `go` limit parsing (clock, increment,
+  `movestogo`, `movetime`, `depth`, `infinite`) with a per-move budget.
+- **Notation** — SAN formatting/parsing, PGN reading/writing, and EPD
+  parsing with a test-suite runner (bundled Win at Chess positions).
+- **Self-play** — a game loop with draw adjudication (fifty-move,
+  insufficient material, threefold repetition).
+
+## Usage
+
+```sh
+# UCI mode
+scala-cli run . -- uci
+
+# self-play a game (maxPlies, depth)
+scala-cli run . -- selfplay 200 6
+
+# perft / search on a position
+scala-cli run . -- <fen|startpos> <depth>
+
+# run the EPD (Win at Chess) suite, N ms per position
+scala-cli run . -- epd 500
+```
+
+Running with no arguments prints usage and a short perft/search demo.
+
+## Tests
+
+```sh
+scala-cli test .
+```
+
+Tests use [munit](https://scalameta.org/munit/) and cover move generation
+and perft, make/unmake and hashing, evaluation and SEE, search heuristics,
+the transposition table, draw detection, SAN/PGN notation, time control and
+the UCI handler.
+
+## Requirements
+
+- scala-cli (Scala `3.8.4`, as pinned in `project.scala`)