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/references/sources.md

@@ -7,3 +7,12 @@
 - **Research doc:** plugins/lint/docs/research/docs/vale/sources.md
 - **Contributing files:** SKILL.md, references/troubleshooting.md
 - **Status:** `extracted`
+
+## house-vale-3-15-2-repro
+
+- **URL:** (house-verified — reproduced against the `vale` binary by a committed test, not an external source)
+- **Description:** Behaviour of Vale 3.15.2 asserted by the committed test (purpose-built fixtures, real `vale` run), where vale.sh documents nothing or documents it wrongly: an unmapped `.mdx` needs an external `mdx2vast` binary (absent, the whole invocation exits 2 with `E100 [lintMDX]`), under `[formats] mdx = md` the HTML-comment suppression form works and the JSX-comment form does not, the `spelling` check's `ignore` paths resolve against `StylesPath` or the working directory but never against the rule file's own directory and fail silently when they resolve nowhere, `ls-config` and the other `ls-*` subcommands report styles and paths but never rules, and the `text.frontmatter.<key>` scope matrix across multi-line YAML forms. Not asserted: the native-MDX column of the suppression table, which needs `mdx2vast` installed.
+- **Research doc:** none
+- **Basis:** tests/test-vale-3-15-2-behaviours.sh
+- **Contributing files:** SKILL.md, references/troubleshooting.md
+- **Status:** `extracted`