- is_varfd: require first char to be letter/underscore, rejecting
  {6d} which is not a valid bash variable name (heredoc_formatting 9)
- is_assignment_word: reject subscripts containing whitespace, so
  arr[0 until ]=$ is not treated as assignment (word_boundaries 2)
- $[...] deprecated arith: new read_deprecated_arith with bracket
  depth tracking replaces read_until_char (word_boundaries 8)
- Trailing \ at EOF: push \\ (literal backslash) in word lexer,
  double-quoted strings, matched parens, param expansion braces,
  and heredoc body (ansi_c_escapes 3, redirect_formatting 3)
- Remove unused read_until_char

6 entries removed from KNOWN_ORACLE_FAILURES.
6 remaining: heredoc_formatting 1 (trailing \n at EOF), ansi_c_escapes
13+18, other 10, heredoc_formatting 8, redirect_formatting 7,
cmdsub_formatting 9.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

- Coproc redirect: parse_coproc now checks if first token after
  coproc is a redirect operator (<<, <>, etc.) and parses it as a
  redirect instead of a word (heredoc_formatting 8, redirect_formatting 7)
- Heredoc delimiter: allow trailing whitespace on delimiter line
  (ansi_c_escapes 18)
- Background ordering: insert trailing & before heredoc content
  on the delimiter line, not after (partial fix for cmdsub_formatting 9)

4 remaining known failures:
- heredoc_formatting 1: trailing \n at EOF in heredoc body
- ansi_c_escapes 13, other 10: bash-oracle control char doubling
- cmdsub_formatting 9: trailing space before ) in cmdsub

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>