vgsp: exclude the 0x40706c read sentinel from the pre-replay CWD-clear predicate; add sema_gl_cwd_clear (A/B knob)

FALSIFIED NOTE (eng16, c271/c272): the eng15 lead that the always-fire
CWD-clear rode on the 0x40706c sentinel and caused the zero-store wall is
FALSIFIED. A/B on the same launch shapes (h=0x1000013 push=0x1200107e4
words=50 / push=0x1200106bc words=53):
- c260 (old code, clear fires 7x/boot on the sentinel): wrote=0.
- c272 (clear fully off, sema_gl_cwd_clear=0): wrote=0 both launches,
  pbfetch=block end, intr0=0 - identical to c260.
- c271 (predicate fixed, knob on): 0 CWD-CLEAR all boot (the sentinel no
  longer arms it; esr=80000000 / grfifo pending counts are clean).
The change still lands because the old predicate was a bug either way:
0x40706c reads 0xbadf5040 always (read-only hardware sentinel; eng15
write test 0x0 -> 0xbadf5040), so every stream3d launch ran a TSG
preempt/resched + MME latch wipe before a noQMD replay that never
re-loads context - an unjustified per-launch context reset.

The zero-store wall is unchanged and now localized: the stream3d lane
only replays X DDX setup-only windows (c272 decode: INC6@0x840
SET_COLOR_TARGET_A(1) VA=0x0c000000 1920x1080 X8R8G8B8, INVALIDATE_*,
INC5@0x180 LINE_LENGTH_IN/OFFSET_OUT - no BEGIN 0x1618 / no draw 0xd74 /
no CALL run), while the pbuffer client's own push entry (0xbeef0100
va=0x4000100 len=262144) is unreadable host-side (rc=-2, channel
unregistered, submissions=0) - so no draw reaches any lane.

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