[PATCH b4 0/2] Detect naked cover letters that lack a 0/N prefix and diffstat

LoreMailbox only recognizes a cover letter when the message carries an
explicit zero counter ([PATCH 0/N]) or, via the in-reply-to fallback in
get_series(), when the thread root carries a diffstat.  A "naked" cover --
one with neither a [PATCH 0/N] subject prefix nor a diffstat in the body --
matches neither test, so it lands in self.unknowns and is dropped, and
LoreSeries.has_cover stays False.

The visible fallout is in shazam -M, which then reports "No cover letter
provided by the author" and links the first patch instead of the real
cover.  This happens whenever an author writes a short prose cover with no
diffstat, e.g. Christoph Hellwig's "don't build bios/contexts over multiple
iomaps" series:

  https://lore.kernel.org/linux-xfs/[email protected]

Patch 1 extends the in-reply-to fallback to also accept the parent as the
cover when it is a non-patch, same-author thread root (in_reply_to is None
and not itself a reply).  Requiring the same author and the thread root
avoids mistaking an unrelated message the series was merely sent
in-reply-to -- a third party's bug report, say -- for a cover letter.

Patch 2 adds positive (naked single-patch cover) and negative (patch sent
in-reply-to a stranger's bug report) fixtures and a test.

Signed-off-by: Christian Brauner (Amutable) <[email protected]>
---
Christian Brauner (2):
      Detect naked cover letters that lack a 0/N prefix and diffstat
      tests: exercise naked cover-letter detection

 src/b4/__init__.py                           | 13 +++++---
 src/tests/samples/naked-cover-bugreport.mbox | 47 ++++++++++++++++++++++++++++
 src/tests/samples/naked-cover-single.mbox    | 44 ++++++++++++++++++++++++++
 src/tests/test___init__.py                   | 32 +++++++++++++++++++
 4 files changed, 132 insertions(+), 4 deletions(-)
---
base-commit: 4217c3e5d3e1eb259626142fd71b91ec6d5e3d1e
change-id: 20260701-work-b4-naked-cover-letter-ee98a040e5c8