Postfix inconsistency across primaries: NUM/STR/list literals reject `.key` (GRAMMAR.md says they take it) while f-strings take postfix undocumented

[InauguralPhysicist]

Found in an adversarial language review at 95a0d76.

docs/GRAMMAR.md's postfix note claims "postfix indexing and dot access apply to NUM, STR, IDENT, list literals, dict literals, and parenthesized expressions". Two divergences:

1. The STR, NUM, and list-literal branches of parse_primary loop only on TOK_LBRACKET (src/parser.c:656, 667, 816) — no .key:

   y is [10, 20].x     # Parse error line 1: unexpected '.' in expression
   s is "ab".foo       # same
   

2. F-strings (unlisted in the note) DO take postfix, because they desugar to a parenthesized expression (src/lexer.c:319):

   print of f"hello"[1]   # → e, rc=0
   

Either give all primaries the uniform postfix loop, or fix the note to match the implementation (and add FSTR to it).