feat: add pygments lexer for lambda discipline

Mirrors the token rules of the language's lexer and highlights builtins
by name, so code on the site reads like it does in an editor.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: nos1dot618

lbd_lexer.py

@@ -0,0 +1,55 @@
+"""Pygments lexer for Lambda Discipline, mirroring src/syntax/token.rs."""
+
+from collections.abc import Iterable, Iterator
+from typing import Any
+
+from pygments.lexer import RegexLexer
+from pygments.token import (
+    Comment,
+    Keyword,
+    Name,
+    Number,
+    Operator,
+    Punctuation,
+    String,
+    Text,
+    Whitespace,
+)
+
+IDENT = r"[A-Za-z_][A-Za-z0-9_']*"
+
+
+class LbdLexer(RegexLexer):
+    name = "Lambda Discipline"
+    aliases = ["lbd"]  # noqa: RUF012
+    filenames = ["*.lbd"]  # noqa: RUF012
+
+    def __init__(self, builtins: Iterable[str] = (), **options: Any) -> None:
+        super().__init__(**options)  # type: ignore
+        self.builtins = set(builtins)
+
+    tokens = {  # noqa: RUF012
+        "root": [
+            (r"\s+", Whitespace),
+            (r"--.*?$", Comment.Single),
+            (r'"([^"\\]|\\.)*"', String),
+            (r"-?[0-9]+(\.[0-9]+)?([eE][+-]?[0-9]+)?", Number),
+            (r"(use|let|in)\b", Keyword),
+            (r"\\|λ", Keyword.Declaration),
+            # A definition: a name at the start of a line followed by `:` or `=`.
+            (rf"^{IDENT}(?=\s*[:=])", Name.Function),
+            (r"(Number|String|Bool|Unit|List)\b", Keyword.Type),
+            (IDENT, Name),
+            (r"->|=|:", Operator),
+            (r"[()\[\],.<>]", Punctuation),
+            (r".", Text),
+        ]
+    }
+
+    def get_tokens_unprocessed(
+        self, text: str, stack: Iterable[str] = ("root",)
+    ) -> Iterator[tuple[int, Any, str]]:
+        for index, token, value in super().get_tokens_unprocessed(text, stack):
+            if token is Name and value in self.builtins:
+                token = Name.Builtin
+            yield index, token, value