docs: finish the 1.4.0 release on the doc surfaces

The 1.4.0 release commit (d19de8c) dated the CHANGELOG heading and nothing
else, so the tag went out with README, ADOPTION-BRIEF and BRIEF still
naming 1.3.0 and the install pin still installing 1.3.0. This bumps the
badge, status line and install pin in README, the status and "as of"
lines in ADOPTION-BRIEF (test count 868 -> 893) and BRIEF's status line.
The tag itself is not moved.

Three claims that went false earlier are corrected against the code:

- README said `resource` is deliberately not on the frontmatter allowlist
  and that a `sources` block list of mappings is refused. Both stopped
  being true in 1.3.0. The paragraph now names the four mapping carriers
  and says `resource` / `usage_window` are admitted inside a `sources`
  entry only.
- README described the `resource` https-allowlist without its scope. It
  covers the top-level `resource` only; `sources[].resource` is never
  URL-validated (measured: `javascript:` there imports with no error).
- LIMITATIONS still listed `tags: [a, b]` among the routes that fail.
  It parses since 1.4.0.

Gates after the edit: 893 passed, coverage exit 0, redos-sweep exit 0,
45 LIMITATIONS entries.

docs/BRIEF.md

@@ -3,7 +3,7 @@
 **A reusable, minimal, dependency-light defensive layer for LLM *ingestion*
 pipelines — the write-time siblings of query-time chatbot guardrails.**
 
-Status: implemented — v1.3.0, exported surface frozen under semver. This document defines what the repo contains
+Status: implemented — v1.4.0, exported surface frozen under semver. This document defines what the repo contains
 and why; the stdlib-only core is built and tested (see `README.md` for usage and
 `docs/PLAN.md` for the build order).