vgsp: CB parser decode fix + pre-replay FE context clear attempt (cwd3 latch survives; zero-store persists)

(a) svc_gl_cb_upload_scan decode now follows the class definitions
(cla16f.h: method 11:0, subchannel 15:13): addr=(hdr&0xfff)<<2, subch=(hdr>>13)&7;
selector B/C swapped (0x2384=B=ADDRESS_UPPER high dword, 0x2388=C=lower,
cla097.h); 0x238c runs apply [offset][constants...] inline data. CB-DIAG
prints subch. Other classes untouched.
(b) Pre-replay CWD poison clear for the stream3d lane: new
gsp_phys_gr_cwd_clear_locked() (checks 0x404490/0x40706c/0x400504 for 0xbadf,
runs the wedge recovery's bounded TSG preempt/resched re-arm + shared latch
wipe) before build_gr_launch; once per launch, stream3d+sema_gr_wedge_recover
gated (CUDA/CE unaffected).

Validation (c250-c254, rich args, ladder armed): the re-arm probes green
('WEDGE-REARM probe rc=0 (engine ALIVE)') but the cwd3 latch does NOT move
('CWD-CLEAR after re-arm ... cwd3=badf5040') and wrote=0 persists (0/1024)
with the 0x1528 trap gone under sema_gl_shader_exc=0 - clause (b) is
falsified as sufficient: the poison is not the TSG/PBDMA context the re-arm
touches. CB-DIAG on these draws shows n3800/n39c8/n2390=0 because bit 12 is
0 on that stream (old/new decodes identical there) - the parser fix stands
on the class definition. X regressed to the DDX DMA wall on these cycles
(DMA-init 0x51 + per-start SIGSEGVs), so clean gl_frames windows were rare.

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