Migrate parallel computing in as.sir() from parallel:: to future/future.apply

Replace parallel::mclapply() and parallel::parLapply() with
future.apply::future_lapply(), enabling transparent support for any
future backend (multisession, multicore, mirai_multisession, cluster)
on all platforms including Windows.

When parallel = TRUE the function now: (1) respects an active
future::plan() set by the user without overriding it on exit, or
(2) sets a temporary multisession plan with parallelly::availableCores()
and tears it down on exit. The max_cores argument controls worker count
only when no user plan is active.

future and future.apply are added to Suggests in DESCRIPTION.

https://claude.ai/code/session_01M1Jvf2Miu6JL4TQrEh1wS8

man/pca.Rd

@@ -32,7 +32,7 @@ pca(x, ..., retx = TRUE, center = TRUE, scale. = TRUE, tol = NULL,
     standard deviations are less than or equal to \code{tol} times the
     standard deviation of the first component.)  With the default null
     setting, no components are omitted (unless \code{rank.} is specified
-    less than \code{min(dim(x))}.).  Other settings for \code{tol} could be
+    less than \code{min(dim(x))}.).  Other settings for tol could be
     \code{tol = 0} or \code{tol = sqrt(.Machine$double.eps)}, which
     would omit essentially constant components.}