fix(interpreter): model bash fork suppression for $SHLVL

[chaliy]

Requested by Михайло · project thread

What changed

• $SHLVL follows bash's fork suppression: when bash would run a command in place (last simple command of a bash -c string, of ( ), $( ), <( ), a background simple command, or exec bash outside ( )), a bash/sh child keeps the parent's level instead of adding one.
• Not lowered where bash forks: { } groups, pipelines, if/case, functions, commands with redirects (outside a sole-command subshell), or when an EXIT/ERR/signal trap is set in that shell.
• bash/sh children no longer inherit the parent's trap handlers (only ignored signals), so bash -c 'trap "echo bye" EXIT; bash -c true' prints bye once.

Why

Level-up roadmap, fidelity step: closes the shlvl-nesting bash-oracle gap (was L-PROC-003).

Before / After (verified)

bash-oracle: match 101 → 102, strict 99 → 100 / 106. floor.txt raised to 102 100.

unset SHLVL
bash -c 'bash -c "echo \$SHLVL"'
bash -c 'x=$(bash -c "echo \$SHLVL"); echo $x; { bash -c "echo \$SHLVL"; }'
bash -c 'trap "echo bye" EXIT; bash -c true'

	output
bash 5.2	1 1 2 bye
this branch	1 1 2 bye
main (oracle case)	nested levels one too high, e.g. 2 for the first line; bye printed twice

Risk

• No new .await points in the hot path; stack for depth_32_no_stack_overflow is unchanged vs main (1849 KiB both).
• Remaining gaps recorded in L-PROC-003: non-shell commands run in place (printenv SHLVL) still see the unlowered value; env bash/timeout bash in place are not lowered; a SHLVL=n prefix on the in-place command wins over bash's lowering.

Checklist

• Failing tests first (shlvl.test.sh, 7 cases), verified against bash 5.2
• fmt, clippy, integration, lib, CLI, bash_comparison, check-okf
• knowledge updated (log, limitations)

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