mpph264enc could not say constrained-baseline, so H.264 was never offered

webrtcsink's codec discovery builds its encoding chain with no output caps, so `force_profile` is
true and it inserts a capsfilter demanding `profile=constrained-baseline` — WebRTC's interoperable
floor. h264parse strips `alignment`, `stream-format` and `parsed` from a caps query but not
`profile`, so that demand reaches mpph264enc's src pad, whose template listed only
`{ baseline, main, high }`. Empty intersection, GstVideoEncoder's sink getcaps returns nothing,
and the failure surfaces upstream as videorate reporting it "could not transform NV12 ... in
anything we support". Discovery then drops H.264 with a warning nobody was reading, VP8 wins by
default, and the session dies in rtpvp8pay.

The element could always produce constrained-baseline; only its static template denied it.
Measured on an RK3566: `mpph264enc profile=baseline ! h264parse` negotiates
`profile=(string)constrained-baseline` on the parser's src pad, because baseline mode turns CABAC
and 8x8 transform off and MPP emits no FMO, ASO or redundant slices. So this widens the template
by one word rather than claiming something new.

Patches now live in a directory per upstream, applied by an `apply_patches` helper and recorded in
the MANIFEST as `patch <project>/<file>`. Two reasons: a patch aimed at the wrong tree failed the
same way a stale one does, and the MANIFEST's flat `patch <file>` line landed next to whichever
.so happened to precede it, which read as provenance for the wrong plugin.

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

patches/README.md

@@ -1,7 +1,12 @@
 # Patches
 
-Applied to the upstream checkout by `scripts/build.sh`, in filename order, and recorded in every
-release's `MANIFEST` so a binary can be traced to the exact source that produced it.
+One directory per upstream — `gst-plugins-rs/`, `gstreamer-rockchip/` — applied to that
+project's checkout by `scripts/build.sh` in filename order, and recorded in every release's
+`MANIFEST` as `patch <project>/<file>` so a binary can be traced to the exact source that
+produced it.
+
+The routing is explicit rather than glob-everything, because a patch aimed at the wrong tree fails
+the same way a stale one does, and the two want different fixes.
 
 **Carrying a patch is a cost, so each one has to say what it buys and how it ends.** A patch with no
 route upstream is a fork with extra steps: it has to be re-cut at every version bump, and the
@@ -12,7 +17,7 @@ itself, rather than silently producing a plugin missing the change it was carrie
 
 ---
 
-## `0001-webrtcsink-no-converter-for-mpph264enc.patch`
+## `gst-plugins-rs/0001-webrtcsink-no-converter-for-mpph264enc.patch`
 
 **What it changes.** `make_converter_for_video_caps` in `net/webrtc/src/webrtcsink/imp.rs` builds
 the chain `webrtcsink` inserts in front of an encoder it selected. It special-cases hardware it
@@ -40,3 +45,38 @@ properties that scale on the VPU, but nothing in `webrtcsink` sets them.
 **How it ends.** Upstream. The `v4l2h264enc` arm on `main` is the same shape for another hardware
 encoder, so the precedent exists; if it is taken, this file is deleted at the next version bump.
 Until then it is re-cut per bump, which `--check` will demand rather than let slide.
+
+---
+
+## `gstreamer-rockchip/0001-mpph264enc-advertise-constrained-baseline.patch`
+
+**What it changes.** One word in `mpph264enc`'s src pad template: its profile list was
+`{ baseline, main, high }` and is now `{ constrained-baseline, baseline, main, high }`.
+
+**Why.** Without it `webrtcsink` cannot offer H.264 on this SoC at all, and says so only at
+`GST_DEBUG=*:WARNING`. Its codec discovery pass builds the encoding chain with no output caps, so
+`force_profile` is true and it inserts a capsfilter demanding
+`video/x-h264, stream-format=avc, profile=constrained-baseline` — WebRTC's interoperable floor.
+`h264parse` strips `alignment`, `stream-format` and `parsed` from a caps query but **not
+`profile`**, so that demand reaches the encoder's src pad, whose template could not satisfy it.
+The intersection is empty, `GstVideoEncoder`'s sink getcaps returns nothing, and the failure
+surfaces far upstream as `videorate` reporting it "could not transform NV12 … in anything we
+support". Discovery then drops H.264 with a warning, VP8 is negotiated instead, and the session
+dies in `rtpvp8pay`. Nothing in the error names the profile.
+
+**Why it is true and not a convenient claim.** `mpph264enc profile=baseline` sets `h264:cabac_en`
+and the 8x8-transform flag off, and MPP emits no FMO, ASO or redundant slices — so the SPS it
+writes really does carry `profile_idc=66` with `constraint_set1_flag`. Measured on an RK3566
+rather than reasoned about: `videotestsrc ! mpph264enc profile=baseline ! h264parse` negotiates
+`profile=(string)constrained-baseline` on the parser's src pad. The element could always produce
+this; only its template denied it. A pad template is a capability set, not current state — the
+same template already advertises all three other profiles regardless of which one the property
+selects.
+
+**The trade-off it makes.** None that we can find, which is itself worth stating: the change only
+widens what the pad may agree to, and the encoder's own src caps still come from its `profile`
+property. A pipeline that asked for `baseline` before still gets it.
+
+**How it ends.** Upstream, at `JeffyCN/mirrors` or whichever Rockchip tree succeeds it. It is a
+one-word capability fix with a reproducer, which is the easiest kind to land; if it is taken, this
+file is deleted at the next pin bump. Until then `--check` demands it be re-cut per bump.

patches/gstreamer-rockchip/0001-mpph264enc-advertise-constrained-baseline.patch

@@ -0,0 +1,11 @@
+--- a/gst/rockchipmpp/gstmpph264enc.c
++++ b/gst/rockchipmpp/gstmpph264enc.c
+@@ -98,7 +98,7 @@ GST_STATIC_PAD_TEMPLATE ("src",
+         GST_MPP_H264_ENC_SIZE_CAPS ","
+         "stream-format = (string) { byte-stream }, "
+         "alignment = (string) { au }, "
+-        "profile = (string) { baseline, main, high }"));
++        "profile = (string) { constrained-baseline, baseline, main, high }"));
+ 
+ static GstStaticPadTemplate gst_mpp_h264_enc_sink_template =
+ GST_STATIC_PAD_TEMPLATE ("sink",