Fourth review found the last import-path bypass: the is_dir() branch returned
None for a name whose dir has no __init__.py, without checking a same-name
sibling file. FileFinder loads a file over an init-less namespace dir, so an
attacker could overwrite a signed foo.py with malicious bytes, mkdir an empty
foo/ next to it (PROVENANCE untouched, still verifies), and any import of that
name ran the unverified foo.py - RCE with only plugins/ write access.
Fix: the dir-with-no-__init__.py branch no longer returns early; it falls
through to the leaf .py hash gate and the non-source refuse check. Invariant:
find_spec returns None for a plugins.* name ONLY where FileFinder would also
find nothing on the same __path__.
Everything else was confirmed sound this round: the owned plugins root, exec of
exact verified bytes (never .pyc/.so), the extension/bytecode refusal, plugin.py
read-once, the provenance signature gate, dev-exemption scoping, and #3/#4.
Symlink, suffix-ordering, cache-lifecycle, and loader-internal angles cleared.
2 regression tests (tampered .py + sibling dir; unsigned .py + sibling dir). All
13 bundled plugins still load under enforcement; 1067 pass, naming green.
beea6c0c9f