bugs: add keywording support

Teach `pkgdev bugs` to file keywording (KEYWORDREQ) bugs alongside
stabilization (STABLEREQ) ones. Each graph node now carries a category
selecting the Bugzilla component, summary suffix, description verb and
keyword form (~arch vs arch); keywordreq bugs are filed version-less, so
the tester/committer picks the best version.

While building a stablereq graph, a dependency that has no keyword at all
on a required arch cannot be stabilized directly, so a keywording bug is
filed for it and the stablereq is chained to depend on it
(parent-stable -> dep-stable -> dep-keyword).

Keywording arches are derived from the keywords other versions of the
package carry (restoring dropped keywords, i.e. rekeywording); when none
can be derived they must be given explicitly after the atom, e.g.
`=cat/pkg-1.2.3 ppc64 riscv`. Requests for masked arches (-arch or -*)
are a hard error, and --keywording is rejected together with
--filter-stablereqs.

This also replaces the assertion that crashed on packages without
derivable keywords with graceful handling.

Resolves: #123
Signed-off-by: Arthur Zamarin <arthurzam@gentoo.org>

Author: arthurzam

NEWS.rst

@@ -2,6 +2,15 @@
 Release Notes
 =============
 
+pkgdev 0.2.16 (unreleased)
+--------------------------
+
+**pkgdev bugs:**
+
+- bugs: support filing keywording (``KEYWORDREQ``) bugs and detecting required
+  keywording of stablereq dependencies, chaining the stable bug to depend on the
+  keyword bug (Arthur Zamarin, #123)
+
 pkgdev 0.2.15 (2026-05-15)
 --------------------------