matrix-synapse: add an explicit msc4306 feature toggle

Expose Synapse's `msc4306_enabled` experimental flag as a first-class MDAD
variable and wire it into `homeserver.yaml` alongside the other experimental
feature toggles.

This makes thread-subscriptions support explicit in playbook configuration,
rather than requiring operators to inject the upstream flag via raw
`matrix_synapse_configuration_extension_yaml`.

The variable intentionally controls only the Synapse feature flag. It does not
change the default `thread_subscriptions` worker count, which remains `0` in the
standard presets. Keeping those as separate choices avoids auto-starting an
experimental worker just because the upstream feature toggle is enabled.

Refs:
- b99a58719b/synapse/config/experimental.py (L600-L602)
- b99a58719b/synapse/rest/client/versions.py (L183-L184)

roles/custom/matrix-synapse/defaults/main.yml

@@ -1423,6 +1423,16 @@ matrix_synapse_max_event_delay_duration: 24h
 # See https://github.com/matrix-org/matrix-spec-proposals/pull/4222
 matrix_synapse_experimental_features_msc4222_enabled: false
 
+# Controls whether to enable the MSC4306 experimental feature ("thread subscriptions").
+#
+# In current Synapse, this also enables the MSC4308 thread-subscriptions extension
+# to Sliding Sync under the same upstream feature flag.
+#
+# See:
+# - https://github.com/matrix-org/matrix-spec-proposals/pull/4306
+# - https://github.com/matrix-org/matrix-spec-proposals/pull/4308
+matrix_synapse_experimental_features_msc4306_enabled: false
+
 # Controls whether to enable the MSC4354 experimental feature (sticky events).
 #
 # This is implemented since Synapse v1.148.0 and can be used by element-call v0.17.0+

roles/custom/matrix-synapse/templates/synapse/homeserver.yaml.j2

@@ -3006,6 +3006,9 @@ experimental_features:
   {% if matrix_synapse_experimental_features_msc4222_enabled %}
   msc4222_enabled: true
   {% endif %}
+  {% if matrix_synapse_experimental_features_msc4306_enabled %}
+  msc4306_enabled: true
+  {% endif %}
   {% if matrix_synapse_experimental_features_msc4354_enabled %}
   msc4354_enabled: true
   {% endif %}