feat(matrix-synapse): Gateway-API as an alternative to the Ingress

Group 3 of the #114 sweep. TWO HTTPRoutes - and the shape is not the one the
issue described.

#114 says this chart renders its Ingresses in a LOOP OVER SEVERAL APPS. It
does not; that is freesailarr. It renders ONE Ingress object with TWO HOST
rules: the homeserver on ingress.domain at "/", and the delegation nginx on
matrix.delegation.serverUrl at "/.well-known/matrix". An HTTPRoute's
spec.hostnames applies to the WHOLE object, so two hostnames that want
different paths and different backends have to be two objects. That is the
only faithful translation, not a stylistic choice.

httpRoute.delegation.hostnames deliberately does NOT fall back to
httpRoute.hostnames. The server name is what REMOTE homeservers resolve and is
a different thing from the homeserver's own hostname; the migration order -
homeserver host first, server name last - depends on telling them apart, and a
chart-global hostname list covering both would erase the distinction.
httpRoute.delegation.parentRefs falls back to httpRoute.parentRefs, which
matters because the server name is usually an apex domain behind a different
Gateway.

ONE THING THIS MODE IMPROVES, and it is the awkward part of Matrix delegation.
Under ingress-nginx two Ingress objects cannot both claim one hostname - the
admission webhook rejects the second - which is why the delegation rule has to
live inside this chart's single Ingress object even though the apex domain
usually belongs to a website. HTTPRoutes attached to the same listener for the
same hostname are merged by the implementation, longest match winning, so the
/.well-known/matrix route can sit beside whatever owns that apex.

WHAT THIS MODE DROPS, MEASURED ON THE LIVE OBJECT ON 2026-10-12 - and ONE OF
THE TWO IS NOT WHAT IT LOOKS LIKE. The live Ingress carried, all of it
operator-supplied through ingress.annotations (this chart sets none of it):

    nginx.ingress.kubernetes.io/proxy-body-size:    50m
    nginx.org/client-max-body-size:                 50m
    nginx.ingress.kubernetes.io/configuration-snippet: |
      more_set_headers "X-Forwarded-For $remote_addr";
      more_set_headers "X-Forwarded-Proto $scheme";
      more_set_headers "Host $host:$server_port";

1. THE BODY SIZE IS THE REAL LOSS. Synapse's media repository takes uploads;
   its own ceiling is max_upload_size in homeserver.yaml, which this chart
   cannot see because the whole config arrives from 1Password. The effective
   limit is the smaller of the two, and in this mode the proxy half becomes
   the GATEWAY'S default - neither number anyone can point at is necessarily
   the one that decides. No counterpart in the Gateway API, so per decision 2
   of #114 it is documented, not rebuilt. nginx.org/client-max-body-size is
   the same limit spelled for NGINX Inc's controller, a hedge across both
   implementations, and it goes for the same reason. (#114's own measured
   table did not list matrix-synapse under proxy-body-size at all; the live
   object has it.)

2. THE SNIPPET IS NOT LOAD-BEARING, which took checking rather than assuming.
   It looks like what Synapse's x_forwarded setting depends on - client IP for
   rate limiting, and federation. It is not: more_set_headers is the
   headers-more module's OUTPUT-header directive ("Replaces (if any) or adds
   (if not any) the specified output headers"), the request-side directive
   being more_set_input_headers. So those three headers are added to the
   RESPONSE TO THE CLIENT; Synapse never sees them, and what it reads with
   x_forwarded: true is the REQUEST's X-Forwarded-For, which ingress-nginx
   supplies itself. The Gateway API's ResponseHeaderModifier could not rebuild
   it anyway (it knows no nginx variables), and should not: NOTES.txt says in
   as many words not to re-create it and not to read its absence as the cause
   of a rate-limiting or federation problem.

   Stated as what it is - a reading of the directive's documented semantics.
   Nothing here observed a Synapse instance with and without the snippet.

NOTES.txt now asks $published (ingress.enabled OR httpRoute.enabled) wherever
it used to ask ingress.enabled - the server-name/serverUrl mismatch warning,
the "clients work fine" aside and the scaled-to-zero paragraph were all really
asking whether anything publishes the release.

Minor: additive, defaults unchanged. Empty helm template diff against
origin/main with ci/matrix-synapse/test-values.yaml.

Part of #114

ci/httproute/matrix-synapse.yaml

@@ -0,0 +1,59 @@
+# Render-only overlay for ci/run-httproute-match-test.sh, applied LAST.
+#
+# Its one job is to put this chart into Gateway mode, because the convention
+# ships with httpRoute.enabled: false and the test would otherwise be green
+# over nothing. A chart that has an overlay here and still renders no
+# HTTPRoute FAILS the test - that is the whole point of the file existing.
+#
+# An overlay and not a change to ci/matrix-synapse/test-values.yaml: the
+# install-test runs exactly ONE configuration per chart, and this chart's
+# fixture installs the StatefulSet with its PVC and the delegation Deployment.
+# Trading its Ingress for a CRD schema check that cyberchef's fixture already
+# performs against the real Gateway-API CRDs would be a bad swap. The install
+# keeps the Ingress; the routes are held here, at render level, which is where
+# their silent failure modes live.
+#
+# ingress.enabled: false is not optional here - ingress.enabled is this chart's
+# routing switch and defaults to TRUE, so without this line the chart aborts
+# its own render with both modes on.
+#
+# WHAT THIS OVERLAY COVERS, WHICH IS BOTH ROUTING OBJECTS - and this chart is
+# the one in group 3 where "both" means two different hostnames, two different
+# paths and two different backends:
+#
+#   - ingress.domain comes from the fixture (matrix.ci.local), so the
+#     HOMESERVER route renders with its hostname from that single source of
+#     truth. httpRoute.hostnames stays empty on purpose: the fallback is the
+#     path every installation will actually use.
+#   - matrix.delegation.enabled is TRUE in the fixture with
+#     matrix.delegation.serverUrl: ci.local, so the DELEGATION route renders
+#     too - on its own hostname, matching PathPrefix /.well-known/matrix, and
+#     pointing at the delegation Service rather than at Synapse. Its
+#     parentRefs fall back to httpRoute.parentRefs below, which is the branch
+#     an installation hits unless the server name sits behind a different
+#     Gateway.
+#   - both routes' backendRefs are checked against the Services of the same
+#     render, and they are DIFFERENT Services. Both references are by PORT
+#     NUMBER rather than by the name "http" the Ingress uses, because
+#     backendRefs takes a PortNumber - so this overlay is what holds those
+#     numbers against the Services'.
+#
+# No URLRewrite filter is rendered by this chart in either mode: the delegation
+# nginx answers on the full /.well-known/matrix/* path, so nothing is stripped
+# and the #118 prefix/redirect assurance has no precondition to meet here.
+#
+# NOT covered here, and it cannot be: that either hostname answers, which for
+# Matrix is also whether FEDERATION works. No Gateway controller runs in CI, no
+# route reaches "Accepted: True", and Matrix federation is HTTPS-only while
+# nothing in this mode renders a certificate. Nor is the body-size limit
+# covered - proxy-body-size: 50m is on the live Ingress and has no counterpart
+# in the specification; that loss is documented in values.yaml and printed by
+# NOTES.txt, and documentation is not something a test can assert.
+ingress:
+  enabled: false
+httpRoute:
+  enabled: true
+  parentRefs:
+    - name: public
+      namespace: gateway
+      sectionName: https