test(lint): back the Vale 3.15.2 behaviour claims with a committed test

The `house-vale-3-15-2-repro` provenance entry claimed behaviours were
reproduced against purpose-built fixtures, but no fixtures existed, so
the earlier commit in this PR removed it. Commit the fixtures.

tests/test-vale-3-15-2-behaviours.sh builds its fixtures in a temp dir
and runs the real Vale. It exits 77 (skipped) when vale is missing or is
not 3.15.2. It asserts the six vale-config behaviours and the vale-run
ones (unmapped .mdx, `vale off` variants, the spelling ignore file, and
the ls-* commands never naming a rule).

Restore the entry in both sources.md files as `Research doc: none` with
`Basis:` naming the test, and re-add its source_keys. Two behaviours are
not asserted: the native-MDX suppression column (needs mdx2vast) and the
`vale sync` row that adds to Packages (needs the network). The wording in
configuration-reference.md and troubleshooting.md now says so.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EGHFJextYtVQseaHPDDhxB

plugins/lint/.apm/skills/vale-run/SKILL.md

@@ -10,6 +10,7 @@ metadata:
   category: lint
   source_keys:
     - context7-websites-vale-sh
+    - house-vale-3-15-2-repro
 ---
 
 ## Gotchas