[MG_IntegrationTest, tools] (P7): the stale-serial red-once is registered on split and spawn only, and the parity gate names it with the other server-side knobs

MGITEST_MAGMA_FORCE_STALE_BUFFER_SERIAL is read by the server. The tcp
arm's server is the lane fixture (scripts/ci/tcp_server_fixture.py),
started once from its own environment, so an entry-level ENVIRONMENT never
reaches it: DirectVulkan.Tcp.StaleSerial.* ran the unforced case under a
name that claimed the knob and could never red (review ID-P7-34, item 1).

The entry is now registered only when the arm is Split or Spawn, and
spawn_lane_parity.py grows a second named asymmetry beside
MAGMA_INPROC_ONLY: MAGMA_NOT_OVER_TCP, key prefixes the reference arm has
and the tcp arm does not by name. It comes off the reference for the tcp
arm alone, so the same entry missing from spawn is still an error. B2 adds
its own knob entries to the same tuple in its tree.

integration-magma-tcp 71 -> 70 entries (68 cases + the two fixture
entries); split 90, spawn 69 unchanged. spawn_lane_parity.py build-split
rc 0; tcp_lane_tools_test.py 6/6.

Integrator merge onto pipe: B2's round-1 mechanism (`mglItestServerEnvKnobArm` in
CMake, `compare_arms(no_tcp=)` in the parity script) already covers this shape, so
`.StaleSerial.` joins MAGMA_SERVER_ENV_KNOB_NO_TCP instead of the separate
MAGMA_NOT_OVER_TCP the package tree introduced.

scripts/ci/spawn_lane_parity.py

@@ -72,8 +72,12 @@ MAGMA_INPROC_ONLY = ("MagmaRunAheadScenario.", "MagmaWireCacheScenario.")
 # These are TAILS, not scenarios - the normaliser above keeps the sub-lane tail for the Magma
 # tier on purpose, so `.MsResolve1.` names the knob entry and `.MsResolve0.` (the default, which
 # does keep its tcp entry) is untouched.
+#
+# P7 wave 2-B3 (ID-P7-34) adds `.StaleSerial.`: MGITEST_MAGMA_FORCE_STALE_BUFFER_SERIAL is read by
+# the server too (VkBufferManager.cpp), so the streamed subdata-then-draw red-once has the same
+# two arms and the same tcp absence. One list, one mechanism, for every server-side knob.
 MAGMA_SERVER_ENV_KNOB_NO_TCP = (".ShaderMip1.", ".ShaderMip2.", ".DepthMip.",
-                                ".DefaultBlitShape1.", ".MsResolve1.")
+                                ".DefaultBlitShape1.", ".MsResolve1.", ".StaleSerial.")
 
 
 def lane_names(build_dir, label):
@@ -103,6 +107,7 @@ def compare_arms(build_dir, tier, labels, inproc_only=(), no_tcp=()):
     it on spawn, which is where it does most of its work.
 
     Returns True on failure, the way main() below counts them."""
+    not_on_arm = not_on_arm or {}
     sets = {}
     for arm, label in labels.items():
         names = lane_names(build_dir, label) - FIXTURE_ENTRIES