`\$` inside double quotes expands as variable instead of literal `$`

[chaliy]

Summary

In double-quoted strings, \$ should produce a literal $ character. Bashkit instead treats the $ as a variable expansion marker after the backslash is stripped, causing unintended variable expansion.

Reproduction

# FAIL — \$ should be literal
echo "\$HOME"
# expected: $HOME
# actual:   /root (or whatever HOME is set to)

# FAIL — with set -u, causes unbound error
set -u
echo "\$nonexistent"
# expected: $nonexistent
# actual:   bash: nonexistent: unbound variable

# FAIL — breaks embedded perl/awk
perl -e "\$x = 42; print \$x"
# expected: perl sees "$x = 42; print $x"
# actual:   bash expands $x before perl sees it

# PASS — single quotes (no expansion, verify no regression)
echo '\$HOME'
# expected: \$HOME
# actual:   \$HOME ✓

Spec tests to add

### backslash_dollar_in_double_quotes
# \$ in double quotes should produce literal $
echo "\$HOME"
### expect
$HOME
### end

### backslash_dollar_with_text
# \$ followed by identifier should not expand
echo "\$foo bar"
### expect
$foo bar
### end

### backslash_dollar_set_u
# \$ should not trigger unbound variable under set -u
set -u
echo "\$undefined_var"
### expect
$undefined_var
### end

### backslash_dollar_in_assignment
# Assignment with \$ should preserve literal
x="\$PATH"
echo "$x"
### expect
$PATH
### end

Where to fix

The lexer at crates/bashkit/src/parser/lexer.rs (around line 1111) correctly converts \$ to a $ character. But then parse_word() at crates/bashkit/src/parser/mod.rs:2539 re-interprets the resulting $ as a variable expansion marker.

The fix needs to distinguish between a $ that came from \$ escaping (should be literal) and a genuine $ expansion marker. Options:

1. Have the lexer emit an escape-sentinel that parse_word() recognizes as literal
2. Have the lexer produce a distinct token/character for escaped-dollar
3. Track escape origin in the character stream so parse_word() can skip expansion

Real-world impact

Critical — \$ in double quotes is extremely common in bash scripts that embed other languages (perl, awk, python). The wedow/harness str_replace tool uses perl -i -0pe "\$old = quotemeta(q{...})" which fails completely in bashkit. Any script using echo "\$var" for literal output also breaks.