`=` was missing from the Pratt table, causing `a = b;` to fail with
"expected `;`, found `=`". Assignment is now BinaryOp::Assign with
binding power (2, 2) — lowest precedence, right-associative — so
`a = b = c` parses as `a = (b = c)`.
Add the LL(1) context-free grammar (GRAMMAR.ebnf), token and syntax
reference (SYNTAX.md), LL(1) verification tool (ll1_check.py), and a
fibonacci example demonstrating the language.
