Default hard ceiling (tokens) on the re-injected ACTIVE SESSION block (#962).
Generous: a true safety cap that normal sessions never hit, so default output
is unchanged while a pathological session can no longer crowd out the task.
Calibrated variant (#539): begin_share < 0.6 means the begin position
empirically under-performs for this client — progress moves to the end
block and the task line is duplicated there (recency rescue for the most
critical item). At the default share the layout is byte-identical to the
uncalibrated version.