[Fix, Test] (MG_Impl, MG_Backend/DirectGLES): DSA by-name texture calls corrupted borrowed-slot memo pairings - process-wide glyph death under Iris

Field report: on Espryt with a BSL Iris pipeline built, every glyph in the
game died - HUD, menu labels, even the vanilla title screen after leaving
the world - while sprites kept rendering. Captured on-device (FCL apitrace
rig), reproduced headlessly on llvmpipe, and pinned with a three-way replay:
the same trace renders full text on raw Mesa desktop GL and on Magma, so
the stream was intact and the execution was Espryt's.

MECHANISM. WithTemporarilyBoundNamedTexture implements the by-name (DSA)
texture entry points by binding the named texture onto the active unit's
real slot, running the bound-texture code, and restoring - without moving
the texture bind generation on either edge. DirectGLES's per-draw texture
sync memo keys on that generation and BORROWS the slot pointer, so a memo
built for texture A kept passing every key while a by-name call had
texture B sitting in the slot: A's backend twin was driven with B's
frontend object, and SyncMipmapsToBackend re-specified A's storage with
B's shape. In the trace, a by-name upload to a BSL 2048x2048 map while
the 16x16 lightmap was bound re-specified the lightmap's GL texture
2048x2048-NULL and back 16x16-NULL. The lightmap exists only as render
output - no glTexSubImage2D ever touches it - so it stayed zero forever,
and rendertype_text (vertexColor = Color * texelFetch(lightmap, ...)),
alpha-discards every glyph. Background quads never sample the lightmap,
which is why only text died.

FIX, class-level, two layers:
- Frontend (shared, closes the same hole for DirectVulkan's generation-
  keyed memos): the temporary bind and the restore each bump the texture
  bind generation (only when the slot actually changed), and the restore
  is an RAII scope guard so a throwing body can no longer leak the
  temporary binding - a second latent bug of the same class. Deliberately
  a generation bump and not a touched-unit note: the high-water mark must
  not chase by-name calls, and a completed bind/restore pair leaves the
  content epoch unchanged, so the cost is an owner-compare re-walk, not a
  memo rebuild.
- DirectGLES defense in depth: both borrowed-pair memos
  (g_unitTextureSyncList, g_fboTextureSyncList) record which frontend
  texture each backend twin was paired with and re-check it before any
  replay (last in the key conjunction, behind the context-id compare). A
  stale pairing now costs a list rebuild instead of silent cross-texture
  storage corruption.

Tests, both red with their own layer reverted:
TextureTest.NamedTextureCallKeepsUnitBindingAccountingCoherent (the
accounting contract) and DirectGLESTextureSync.UnitMemoRefusesToDriveA-
TwinFromAnotherTexture (the corrupting sequence shape against a mock GLES
table, asserting the resident texture's storage is never re-specified).
595/595 unit at default and with the async kill switch. Replay evidence:
the captured BSL ESC-menu trace renders all text through Espryt post-fix,
byte-comparable to the Mesa-direct and Magma replays; the no-shaderpack
control is unchanged. A trace fixture wiring this scene into CI follows
in a separate commit.

MobileGL/MG_Test/Texture/TextureTest.cpp

@@ -2835,3 +2835,53 @@ TEST_F(TextureTest, BindSamplerRejectsUnitsBeyondMaxCombinedTextureImageUnits) {
     MG_Impl::GLImpl::DeleteSamplers(1, &sampler);
     DrainPendingGlErrors();
 }
+
+// The DSA by-name entry points are emulated by temporarily binding the named texture onto the
+// active unit's slot for its target, running the classic bound-texture code, then putting the
+// previous binding back. For as long as the emulated call runs, that swap is a REAL change to
+// which texture is bound at that unit, so both transitions have to move the texture bind
+// generation.
+//
+// They used to move nothing. Backends memoise per-unit work keyed on the bind generation and
+// BORROW the binding slot (they hold a pointer to the slot's shared_ptr, not a copy), so a memo
+// built while texture A sat in the slot stayed "valid" while B was temporarily in it - and the
+// backend then drove A's backend twin from B's frontend state, re-specifying A's backend storage
+// with B's shape. Any content A only ever had on the GPU was gone. That is what blanked
+// Minecraft's lightmap when Iris uploaded to a BSL shadow map: the text shader multiplies by the
+// lightmap, so `if (color.a < 0.1) discard` then threw away every glyph in the process.
+TEST_F(TextureTest, NamedTextureCallKeepsUnitBindingAccountingCoherent) {
+    GLuint names[2] = {};
+    MG_Impl::GLImpl::GenTextures(2, names);
+    const GLuint boundName = names[0];
+    const GLuint namedName = names[1];
+
+    MG_Impl::GLImpl::ActiveTexture(GL_TEXTURE0);
+    // Instantiate both as 2D objects, then leave `boundName` on the unit.
+    MG_Impl::GLImpl::BindTexture(GL_TEXTURE_2D, namedName);
+    MG_Impl::GLImpl::BindTexture(GL_TEXTURE_2D, boundName);
+    ASSERT_EQ(MG_Impl::GLImpl::GetError(), GL_NO_ERROR);
+
+    auto& slot = MG_State::pGLContext->GetTextureUnitObject(0).GetBindingSlot(TextureTarget::Texture2D);
+    const auto boundObject = slot.GetBoundObject();
+    ASSERT_NE(boundObject, nullptr);
+    ASSERT_EQ(boundObject->GetExternalIndex(), boundName);
+
+    // TextureParameteriv is one of the by-name calls that is emulated by binding: it reaches
+    // WithTemporarilyBoundNamedTexture, unlike the scalar TextureParameteri, which edits the
+    // object directly and never touches a unit.
+    const Uint64 base = MG_State::pGLContext->GetTextureBindGeneration();
+    const GLint maxLevel = 0;
+    MG_Impl::GLImpl::TextureParameteriv(namedName, GL_TEXTURE_MAX_LEVEL, &maxLevel);
+    EXPECT_EQ(MG_Impl::GLImpl::GetError(), GL_NO_ERROR);
+
+    // The emulation put `namedName` on the unit and took it off again. A generation-keyed memo
+    // must be able to see that the slot it borrows was not stable across the call.
+    EXPECT_GT(MG_State::pGLContext->GetTextureBindGeneration(), base)
+        << "a by-name texture call swapped a live unit binding without moving the bind generation";
+    // ...and the application-visible binding is exactly what it was before the call.
+    EXPECT_EQ(slot.GetBoundObject(), boundObject);
+
+    MG_Impl::GLImpl::BindTexture(GL_TEXTURE_2D, 0);
+    MG_Impl::GLImpl::DeleteTextures(2, names);
+    DrainPendingGlErrors();
+}