vgsp: DDX-family USERD latch lifetime - served-latch deferral + live re-probe + FB backoff

The X DDX's push-on-demand generations allocate a new USERD control block
every 11-24 s, but the 15 s no-advance judge dropped the latch that had just
passed the DMA test, and the next scan adopted a frozen FB lookalike
(put=2 get=0) - the retry window then closed with 'Failed to idle DMA'.
For the DDX/kick handle families only (0x01xxxxxx / 0xbfef00xx; SYS/CE/UVM
untouched):
- svc_userd_ddx_family() gates all of it;
- a latch that HAS delivered decoded entries (submissions advanced since
  adoption, no contradiction) is DEFERRED from rejection (bound 32; timer
  reset) instead of dropped;
- before any latch decision (and on a 2 s cadence while unconfirmed) the
  content scan re-probes the live path and switches the latch in place to a
  newly validated block (svc_userd_ddx_reprobe; near-empty page + ring-entry
  validation + PUT-step liveness are the ownership proof);
- after a rejection the FB shape scan backs off 6 s so the RAM scan wins the
  window; the DDX re-probe is exempt from the retired slice budget
  (rejects < 255).

Validated (c141, rich r1624 args, tick): X ScreenInit passed with ZERO
'Failed to (idle|initialize) DMA' / push-buffer / AddScreen lines; the
channel decoded a 5090+-entry burst and served across block moves
(latch SWITCHED 0x24128e100 -> 0x103774100 -> 0x122309d00, uok=1 held).
Known residual (next refinement): the switch is not yet forward-only/
rate-limited, so lookalike blocks can oscillate at ~2 s and starve the
client; make the switch require nput > latch_put and a >=10 s cadence (or
stop switching once defers>0), then the real block wins.

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