JOURNAL-parse

Design decisions

Grammar: classic 3-level recursive descent:
- expr → term (('+' | '-') term)* (lowest precedence)
- term → unary (('*' | '/') unary)* (medium)
- unary → '-' unary | primary (right-recursive for stacked unary)
- primary → NUMBER | '(' expr ')'
Left associativity falls out naturally from the while loops in _expr and _term.
Unary is right-recursive (- - 5 → nested Unary) as is conventional.
Empty input detected early at parse() entry before any descent.
Trailing tokens detected after _expr() returns, before EOF check.