vgsp-sema: retryable page-directory discovery from fresh guest-truth pairs (ISSUE-0001)

svc_pd_discover() latched svc_pd_tried before its first scan, so a call
that ran before the client's page tables existed (or before enough
verified push-buffer pairs were known) disabled the one kernel-native
source that translates pool/push windows the content-scanned map and the
published directories miss, for the rest of the boot. That source is what
svc_lookup_trusted() and svc_pool_pa_resolve() fall back to when the
userspace watcher pins race.

Make discovery retryable and evidence-driven: an attempt that cannot run
does not latch, a scan runs only when the sample-set fingerprint changed,
and the total is capped at SVC_PD_ATTEMPTS_MAX per boot - the same order
as the old per-pairing re-arm, which this supersedes (the stale re-arm in
svc_pb_pair is removed).

Evidence: ISSUE-0001, SA-STALL 3.2/3.4 (the deferred pool release of the
last CE push with no translation when watchers race).

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