v1.27.1 — fixes the 1.27.0 apply crash

Supersedes v1.27.0, which has been withdrawn. If you installed 1.27.0, upgrade: every apply in it crashed with name 'source' is not defined.

Fixed

• apply crashed with name 'source' is not defined. The 1.27.0 refactor that moved the stream-pool build above the placeholder decision dropped the line binding source, which the logo resolver still uses.
• New gate (tests/test_no_undefined_names.py): pyflakes over every .py, failing on undefined names — the class that caused this. It carries a positive control and an instrument check that plants an undefined name and asserts the detector flags it.
• 23 pre-existing undefined names in tests/ (missing typing imports behind PEP 526 local annotations).

Everything in the 1.27.0 notes applies to this release: Preferred languages, Demote/Exclude stream groups, and provenance ordering.