executor: a lone $ is a literal, not the shell pid (#672)

[berrym]

Closes #672.

The defect

$$ is the process ID. A single $ introduces nothing — there is no parameter named "" — so the only coherent reading is the character itself. An explicit special case returned the PID:

$ lush -c 'echo $'
71681                    # bash, zsh, dash: $
$ lush -c 'v=$; echo "[$v]"'
[71681]                  # stores a pid

Every other position was already correct — a$, "$", '$', 5$, $%, cost: 5$ all produced the literal. That is why it only showed when the $ stood alone as an unquoted word, and why it survived so long.

Curation

• Baselines: bash, zsh and dash all print the literal.
• lush's own model: same answer, derived independently.
• Can lush do better than all three? No. A lone $ has one sensible meaning and this is it. Recorded rather than left implicit; mode-invariant, gates nothing.

On the audit blind spot

The issue noted the CST audit could not flag this, because the legacy and CST paths were equally wrong — there was no divergence to catch. Both were verified to agree after the fix: default CST route, LUSH_WORD_CST=0, and under LUSH_WORD_CST_AUDIT=1 (no abort).

Verification

• tests/integration/test_dangling_dollar.c — 24 checks: the lone $ in eight positions (whole word, before another word, assigned, doubled, through a pipe, inside a substitution, as a printf argument, in a for list); $$ still a pid, asserted by shape since it has no fixed value; $? and $# untouched; the six positions that were already correct; and six where a $ sits adjacent to a real expansion ($v$, $(printf x)$, $((1+1))$, ${HOME:+x}$) that must not be disturbed.
• 9 of 24 fail against the parent build, with a control proving the binaries differ.
• Full suite 196/196. ASan 65/65; the new test 1/1 under ASan.

[codecov]

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