[MG_Backend, MG_Impl, MG_Util, MG_Test, MG_IntegrationTest, docs] (P7): the server publishes kCapResidentSubData off its own wire resource table

CONTRACT-P7 §5.4, OQ-10. InitServerRoleCommon computes the whole capability mask and
never ORed this bit, so MGPipeResourceOpsHaveSubDataResident answered false under every
transport and BOTH backends were pinned to the ordered in-place host memcpy - which made
Magma's SubDataResident arm (VkBufferManager.cpp's g_vulkanWireResourceOps) dead code on
the wire even though the arm has been there since P5f.

The bit is derived from g_resourceOpsAtStep2->SubDataResident, i.e. from the pointer this
function already pinned at step 2 and that step 5 already refuses to see swapped - never
from ActiveBackendType. That is ID-39's lesson and not a style preference: a mask is a
statement about THIS backend's registered table, and a bit derived from the enum would
keep claiming the capability for a build, a flavour or a future backend whose table does
not carry the arm.

`rsd=` joins the emit[] bracket on the PipeStats summary line, counted at the emitter once
per buffer_subdata_resident record rather than once per call, because a write past the
content chunk cap is cut into several records. It is the only observable that separates
the resident arm from the memcpy beside it: the bytes in the buffer are identical either
way, which is exactly how the capability stayed unpublished for a phase with every lane
green.

WHAT THIS DOES NOT YET PROVE, and R-6 is the reason rather than an omission. §5.4 asks for
a white-box case showing opcode 49 on the wire. None can exist at this tree: the resident
arm is reached only from a GPU-RESIDENT store, residency comes only from
AdoptPersistentMap, and every persistent-map acquisition declines under split by R-6
(PipeApply.cpp:2331 "A SPLIT BUILD RUNS AT TIER T2 AND DECLINES EVERY ACQUISITION,
ALWAYS"; T0/T1 are P11's and AdoptTierIsEmulate is a named Fatal for them). So the
capability is wired AHEAD of its consumer, deliberately, and `rsd=` is what will show the
records the day P11 lands a tier that adopts. Filed for the integrator in
notes/p7/magma-c.md.

What IS proved is both halves of the bit itself. RemoteClientTest's Magma bootstrap case
now asserts the published bit EQUALS `ops->SubDataResident != nullptr` - the two compared
to each other rather than both to true, so the case also covers a backend without the arm
on the day one exists - and a new CtWireScenario case reads the bit out of the client's
own caps mirror, through the same accessor the frontend reads, on both backends and all
three transports.

RED-ONCE, both levels. Deleting the OR reds
CapsMirrorTest.MagmaTransportPublishesRealBufferConsumersWithoutRunAhead
(RemoteClientTest.cpp:692, "Which is: false / Which is: true") and all SIX arms of
DirectGLES/DirectVulkan x {Split,Spawn,Tcp} Ct.TheServerPublishesTheResidentSubData
CapabilityFromItsOwnTable ("the client's caps mirror did not receive
kCapResidentSubData"). Restored: green.

Also carries the P4a death-helper comment from the previous commit's sweep: it no longer
says Magma installs no death table, because it now does and still frees no slot.

MobileGL/MG_Test/Wire/RemoteClientTest.cpp

@@ -705,6 +705,15 @@ TEST(CapsMirrorTest, TheReadAccessorsStillAnswerFromThePlaceholder) {
     CapsMirror mirror;
     EXPECT_FALSE(mirror.Valid());
     EXPECT_EQ(mirror.Generation(), 0u);
+    // (F1 made ServerConsumes on a placeholder a refusal, so the two decision reads that used to
+    // sit here belong to the case above; the READ accessors below are what this case pins.)
+    // P7 wave 2 package C, OQ-10: the NEGATIVE half of the resident-sub-data pair. It used to
+    // be vacuous - nothing in the tree ever set kCapResidentSubData, so every reading of it was
+    // false - and MagmaTransportPublishesRealBufferConsumersWithoutRunAhead below is the
+    // positive half that makes this one mean "a mirror with no snapshot withholds the bit"
+    // rather than "the bit does not exist". Withholding is the SAFE direction: the client then
+    // keeps the ordered in-place host write (BufferObject::LandBytesIntoResidentStore) instead
+    // of emitting opcode 49 at a server that might have no arm for it - ID-39 in miniature.
     EXPECT_FALSE(mirror.HasCap(kCapResidentSubData));
     EXPECT_EQ(mirror.CallMask(), 0u);
     EXPECT_EQ(mirror.Backend(), BackendType::Unknown);
@@ -789,6 +798,24 @@ TEST(CapsMirrorTest, MagmaTransportPublishesRealBufferConsumersWithoutRunAhead)
             EXPECT_NE(ops->MapPersistent, nullptr);
             if (ops->MapPersistent) EXPECT_EQ(ops->MapPersistent({7, 1}, 64, nullptr), nullptr)
                 << "Magma must not donate a server address as a client persistent map";
+            // P7 wave 2 package C, OQ-10 (CONTRACT-P7 §5.4): THE TABLE DECIDES THE BIT, and
+            // this is the assertion that says so rather than asserting a constant. Magma
+            // registers a SubDataResident arm (VkBufferManager.cpp's g_vulkanWireResourceOps),
+            // so the published mask must carry kCapResidentSubData - and it must carry it
+            // BECAUSE of the arm, which is why the two sides are compared to each other
+            // instead of both to `true`. Written this way the case also covers the backend
+            // that does NOT have the arm, on the day one exists: no enum is consulted here,
+            // and deleting the `|=` in MG_Backend/Init.cpp reds exactly this line (red-once).
+            EXPECT_EQ((server.CallMask() & static_cast<Uint64>(kCapResidentSubData)) != 0,
+                      ops->SubDataResident != nullptr)
+                << "kCapResidentSubData must be published from the server's own wire resource "
+                   "table (ID-39: never from the backend enum). Without it "
+                   "MGPipeResourceOpsHaveSubDataResident answers false under every transport "
+                   "and both backends fall back to the in-place memcpy, which makes Magma's "
+                   "SubDataResident arm dead code on the wire";
+            EXPECT_NE(ops->SubDataResident, nullptr)
+                << "Magma's wire resource table lost its resident sub-data arm; the bit above "
+                   "would then correctly go dark and opcode 49 would stop crossing";
         }
         EXPECT_TRUE(ClientSessionInstance().RunAheadArmed());
         ClientSessionInstance().Stop();