stack-release: refuse unreadable drop-ins and quoted units, recheck the bus after a held read, gate on the target copy (row 166, #1663)

drop_in_override refuses drop-in-override when a unit path, a drop-in
directory or a .conf exists but can't be read, as install_guard in
bus-service.sh would after the stop. A unit path that isn't absolute
refuses too.

plan_store and the rollback --restore event count check is-active again
after reading the store the running bus holds. If the bus stopped
meanwhile, --plan prints "store schema: unknown" with no migration lines,
and rollback refuses restore-mismatch before anything stops.

The gate's copy-check step compares the check's bus line with the copy
this run installed and names both when they differ.

read_units returns 1 when the bus Environment= line or either unit's
ExecStart argv holds a quote or a backslash, so activate and rollback
refuse unit-flags-drift, adopt refuses check-failed and status prints
that it cannot read the units. Nothing parses systemd quoting.

Tests T28 to T31 in packages/cli/tests/stack-release.test.mjs.