feat: implement `set -a` (allexport)

[chaliy]

Problem

set -a (allexport) is parsed and the flag SHOPT_a is stored, but the behavior is not implemented:

1. option_name_to_var() in builtins/vars.rs:37-48 does not map "allexport", so set -o allexport is silently ignored
2. Nothing checks SHOPT_a during variable assignment — new/modified variables are never auto-exported

This is used by frameworks like wedow/harness to load session environment files:

set -a
source "${HARNESS_SESSION}/.env"
set +a

Expected behavior

When set -a is active, every variable assignment (via =, read, for loop variable, etc.) should also mark the variable as exported. set +a disables this. set -o allexport and set +o allexport should work as aliases.

Test cases

Basic allexport

set -a
FOO="bar"
BAZ="qux"
set +a

AFTER="not-exported"

# Check export status
env_output="$(env)"
echo "${env_output}" | grep -c "^FOO=bar$"
echo "${env_output}" | grep -c "^BAZ=qux$"
echo "${env_output}" | grep -c "^AFTER="
# Expected stdout:
#   1
#   1
#   0

allexport with source

cat > /tmp/vars.env <<'EOF'
DB_HOST=localhost
DB_PORT=5432
EOF

set -a
source /tmp/vars.env
set +a

cat > /tmp/check-env.sh <<'SCRIPT'
#!/usr/bin/env bash
echo "host=${DB_HOST:-unset}"
echo "port=${DB_PORT:-unset}"
SCRIPT
chmod +x /tmp/check-env.sh

/tmp/check-env.sh
# Expected stdout:
#   host=localhost
#   port=5432

set -o allexport / set +o allexport

set -o allexport
X="hello"
set +o allexport
Y="world"

export -p | grep -c "declare -x X="
export -p | grep -c "declare -x Y="
# Expected stdout:
#   1
#   0

Short flag in combined options

set -eau
MY_VAR="test"
set +a

export -p | grep -c "declare -x MY_VAR="
# Expected stdout: 1

allexport applies to read builtin

set -a
echo "value" | read -r MY_READ_VAR
set +a

# MY_READ_VAR should be exported
export -p | grep "MY_READ_VAR" | head -1
# Expected stdout: declare -x MY_READ_VAR="value"

allexport applies to for loop variable

set -a
for item in alpha beta; do
  : # item should be exported on each iteration
done
set +a

export -p | grep -c "declare -x item="
# Expected stdout: 1

Variables assigned before set -a are not retroactively exported

BEFORE="exists"
set -a
DURING="new"
set +a

export -p | grep -c "declare -x BEFORE="
export -p | grep -c "declare -x DURING="
# Expected stdout:
#   0
#   1

Implementation hint

1. Add "allexport" => Some("SHOPT_a") to option_name_to_var() in builtins/vars.rs
2. Add "allexport" entry to SET_O_OPTIONS array for display
3. In variable assignment paths, check if SHOPT_a is "1" and if so, also insert into self.env (the export set)
   • Key locations: process_command_assignments, execute_local_builtin, for loop variable assignment, read builtin side effects

[chaliy]

This feature is already fully implemented. Verified all test cases from the issue:

• set -a / set +a correctly auto-exports variables assigned while active
• set -o allexport / set +o allexport work as aliases
• Allexport applies to sourced files, for-loop variables
• Variables assigned before set -a are not retroactively exported
• option_name_to_var() already maps "allexport" to "SHOPT_a"
• set_variable() already checks SHOPT_a and exports to self.env

Added 5 regression tests in allexport_tests.rs — all pass.

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