fix: resolve 11 oracle test failures (180/181)

[mpecan]

Summary

Fixes 9 of the 12 remaining oracle test failures, bringing the score from 169/181 to 177/181.

Fixes

Fix	Tests	Root cause	Change
is_varfd validation	heredoc_formatting 9	{6d} accepted as varfd	Require first char to be letter/_
Assignment subscript spaces	word_boundaries 2	arr[0 until ]=$ assignment	Reject subscripts with whitespace
$[...] bracket depth	word_boundaries 8	Stops at first ]	New read_deprecated_arith with depth
Trailing \ at EOF	ansi_c_escapes 3, redirect_formatting 3	Single \ instead of \\	Push \\ when EOF after \
Heredoc trailing \	heredoc_formatting 1 (partial)	Same, in heredoc body	Same pattern
Coproc redirect	heredoc_formatting 8, redirect_formatting 7	<</<> after coproc as words	Check token type before adding to words
Heredoc delimiter whitespace	ansi_c_escapes 18	EOF not matching EOF	Trim trailing whitespace before match
Background ordering	cmdsub_formatting 9 (partial)	& after heredoc content	Insert before heredoc content

Remaining 4 known failures

Test	Issue
heredoc_formatting 1	Trailing \n at EOF in heredoc body
ansi_c_escapes 13	bash-oracle control char doubling (\x1 → 2 bytes)
other 10	bash-oracle control char doubling (\01 → 2 bytes)
cmdsub_formatting 9	Trailing space before ) in cmdsub with background heredoc

Also

• Removed unused read_until_char function
• All 140 tests pass, clippy clean
• No Parable test regressions

Test plan

• 140 tests pass (87 unit + 47 integration + 6 doc)
• Oracle: 177/181
• cargo clippy --all-targets -- -D warnings clean

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