Correct the claim: Radxa's plugin is not decode-only

It contains every encoder — strings on its .so lists mpph264enc, mpph265enc,
mppjpegenc and mppvp8enc. The earlier claim here was wrong, and it was the
headline reason given for building our own, so it needs correcting rather than
quietly dropping.

What actually happened is one cause behind four symptoms, and this build
surfaced the fourth: an MPP plugin registers its decoders unconditionally but
*probes MPP* before registering its encoders. With /dev/mpp_service at 0600
root:root the probe fails silently. So mpi_enc_test wrote an empty file and
exited 0; Radxa's deb looked decode-only; a third-party deb installed and still
showed no mpph264enc; and this repository's own CI build lists only mppjpegdec
and mppvideodec, because a container has no such node either. That last one is
expected, not a failure, and the README now says so where somebody reading a
green build log will find it.

The reasons to build that survive: no libx11-6, since rkximage and kmssrc are
disabled; a pin we control, recorded in every release; and this repository has
to exist for gst-plugins-rs regardless, which no archive packages at all — so
the MPP plugin riding along costs almost nothing.

Assisted-by: Claude:claude-opus-5[1m]

pins.env

@@ -11,9 +11,16 @@
 # `rockchip-linux/gstreamer-rockchip` — the homepage both Radxa's and every third-party deb name
 # — is a 404 now. `JeffyCN/mirrors` is the live mirror, same maintainer (Jeffy Chen).
 #
-# Why build it at all: Debian has no Rockchip encoder in any suite, and Radxa's
-# `gstreamer1.0-rockchip1_1.14-4` is **decode-only** — measured on a Zero 3W, it provides
-# `mppvideodec` and `mppjpegdec` and nothing else. 1.14.4 predates the encoders.
+# Why build it at all: Debian has no Rockchip encoder in any suite. Radxa publish a
+# `gstreamer1.0-rockchip1_1.14-4` that **does** contain every encoder — `strings` on its .so lists
+# `mpph264enc` and the rest — so "theirs is decode-only" is not the reason, and an earlier version
+# of this comment claiming so was wrong. On a board it *appears* decode-only, and the cause is the
+# permission trap below, not the build.
+#
+# The reasons that survive: this build drops `rkximage` and `kmssrc` and so needs no `libx11-6`;
+# the pin is one we control and a release records it; and this repository has to exist for
+# `gst-plugins-rs` regardless, which no archive packages at all — so the MPP plugin riding along
+# costs almost nothing.
 # **Not the branch tip, and this is the interesting part of the pin.**
 #
 # Branch HEAD (dcbcd645, 2026-05-21) does not compile against the MPP Radxa ships. Commit