pm imported runtime_state's private helpers (_lock, _atomic_bytes, _bytes,
_digest) at a dozen sites while runtime_state imports pm.environments at
module top. The primitives are pm's: move them into the stdlib-only
pm.filesystem as lock_fd, durable_write_bytes, read_bytes_or_none and
file_digest, and repoint every pm caller.
runtime_state keeps the private names only as import aliases: it still
calls them through its own globals, and pre-PM updaters load them by these
names mid-swap (tests/compat/old_updater_surface.json).
Boot-subset test fixtures now copy pm/filesystem.py, since runtime_state
imports it at process boot; worker-injection tests patch the name
pm.publication now reads.
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