Engine: drop the layer-loop memory flush in preset M

The ten network layer dispatches only need dispatch serialization, not a
global memoryBarrier flush: each layer's cross-block traffic goes through
its own buffers, already published by the preceding stage's flush. An
empty compute->compute pipeline barrier keeps the ordering and removes
the flush cost: whole-frame median 2515.0 -> 2431.7 us at Balanced
(1488x837 -> 2560x1440), 3818.9 -> 3773.3 at Quality, 1892.6 -> 1810.6
at Performance, byte-identical to the captured launches (chain test
10/10 layers 100.0%, frame test inf dB, 8- and 16-frame outputs match).

Also documents this session's discarded probes (intra-frame barrier
relaxation, colSum subgroup shuffle, downsample BLOCK_Y 32, expansion
EO stride 24, cross-frame pipelining analysis, the NO_ROUND 7.0%
rounding-free ceiling) and adds the round_half.glsl include guard.

engine/round_half.glsl

@@ -3,6 +3,8 @@
 // gfx1201, and so is the packHalf2x16 round-trip (which is round-toward-zero anyway): the rounding only happens
 // once the binary16 value lands in binary16 memory. Scalar f32 -> f16 -> f32 casts are folded the same way,
 // notably for filtered recurrent features, hence the integer helper below.
+#ifndef D4R_ROUND_HALF_GL
+#define D4R_ROUND_HALF_GL
 // Cooperative matrices are composites of implementation-dependent components: every invocation owns and may
 // subscript a fixed number of them (length(), 8 for a 16x16 subgroup-scope accumulator on a 32-lane wave).
 float roundHalfValue(float v) {
@@ -40,3 +42,5 @@ vec4 roundHalfValue(vec4 v) {
         (m)[rh_i_] = float(storage[gl_LocalInvocationID.x * 8u + uint(rh_i_)]); \
     subgroupMemoryBarrierShared(); \
 }
+
+#endif