vgsp-sema: keep push-buffer base discovery alive until the CE windows drain (ISSUE-0001)

svc_pb_discover() ran with a flat 12-pass budget for the whole boot and
then retired, even when a registered CE channel had no content-discovered
push-buffer base yet. A channel that appeared after the budget was spent
(including one reappearing with a new client session) then had no paired
window, so every push went through the slower per-page rescue and the
userspace watcher pin stream.

Stop the pass only after SVC_PB_IDLE_PASSES consecutive passes with no
new base AND with every registered CE channel either paired or having a
base recorded for its release slot, with a hard cap (SVC_PB_PASS_MAX) so
a channel that genuinely has no base page cannot scan forever. The 3 s
rate limit and chunked walk keep the service thread responsive; the
per-pass log now carries the idle count.

Evidence: ISSUE-0001; v2-r1 counter freeze (pb=6/12, see
backups/evidence-20261002-victory/11-hostdmesg-v2-r1.log) and the SA-STALL
class where the CE window was reached only after discovery had stopped.

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