Redo ERC truncation and /CLEAR hook mechanism

Redo ERC truncation and /CLEAR hook mechanism

* etc/ERC-NEWS: Mention option `erc-truncate-padding-size'.
* lisp/erc/erc-fill.el (erc-fill-wrap-mode, erc-fill-wrap-enable)
(erc-fill-wrap-disable): Manage membership in the `erc--clear-function'
advice stack for own function that massages a buffer's oldest inserted
message, post truncation.
(erc-fill--wrap-massage-initial-message-post-clear): New function.
* lisp/erc/erc-log.el (erc-log-mode, erc-log-enable): Don't add
`erc-save-buffer-in-logs' to `erc--pre-clear-functions'.  Use local
advice around common interface variable instead, as noted below.
(erc-log-disable): Likewise, don't remove `erc-save-buffer-in-logs' from
`erc--pre-clear-functions'.
(erc-log-setup-logging): Add `erc-log--save-on-clear' to
`erc--clear-function'.
(erc-log-disable-logging): Remove `erc-log--save-on-clear' to
`erc-clear-function'.
(erc-save-buffer-in-logs): Abort when `erc--insert-marker' is non-nil.
(erc-log--save-on-clear): New function, a thin wrapper around
`erc-save-buffer-in-logs', adapting it to the `erc--clear-function'
advice interface.
* lisp/erc/erc-stamp.el (erc-stamp-mode, erc-stamp-enable): Don't
add `erc-stamp--reset-on-clear' to `erc--pre-clear-functions'.
(erc-stamp-disable): Don't remove `erc-stamp--reset-on-clear' from
`erc--pre-clear-functions'.
(erc-stamp--find-insertion-point): Account for initial position being
`bobp'.
(erc-stamp--defer-date-insertion-on-post-modify): Accommodate the rare
non-list `erc-insert-post-hook' when shadowing.
(erc-stamp--setup): Add and remove `erc-stamp--reset-on-clear' to and
from `erc--clear-function' advice stack.
(erc-stamp--redo-right-stamp-post-clear): New function.
(erc-stamp--update-saved-position): Remove unused function.  This was
originally added along with `erc-stamp--reset-on-clear' as part of
bug#60936.
(erc-stamp--reset-on-clear): Expect end of truncation boundary to be at
`erc-insert-marker'.  Rework to use new `erc--clear-function' interface
and run on `erc-timer-hook' instead of `erc-insert-done-hook'.
* lisp/erc/erc-truncate.el (erc-truncate-padding-size): New option to
help tamp down on disruptions when reading scroll back caused by overly
frequent truncation.
(erc-truncate-enable, erc-truncate-disable): Add and remove
`erc-truncate--setup' to and from `erc-mode-hook', and run it when
needed.
(erc-truncate--buffer-size): New variable.
(erc-truncate--setup): New function.
(erc-truncate-buffer-to-size): Guard execution with
`erc-truncate--padding-size' and `erc--inhibit-clear-p'.  Reflow for
readability, removing obsolete comments.  Call hooks with marker instead
of buffer position, as per the new `erc--clear-function' interface.
(erc-truncate-buffer): Defer execution to `erc-timer-hook' when running
post-insertion via a response handler.
(erc-truncate--inhibit-when-local-and-interactive): New function.
* lisp/erc/erc.el (erc-mode): Add `erc--skip-past-headroom-on-clear'
to `erc--clear-function' in all ERC buffers.
(erc--with-spliced-insertion): Account for marker being `bobp'.
(erc--insert-before-markers-transplanting-hidden): Make more robust by
accommodating initial `point' possibly being `bobp'.
(erc--clear-function): New variable, a function-valued local-advice
interface to replace `erc--pre-clear-functions'.
(erc--pre-clear-functions): Remove unused variable.
(erc--skip-past-headroom-on-clear): New function.
(erc--inhibit-clear-p): New variable.
(erc-cmd-CLEAR): Call hooks with markers instead of position.  Signal
`user-error' when `erc--inhbiit-clear-p' is non-nil.
* test/lisp/erc/erc-scenarios-log.el (erc-scenarios-log--clear-stamp)
(erc-scenarios-log--cmd-clear/date-stamps): Rename former to latter,
update assertions, and use common helper.
(erc-scenarios-log--cmd-clear/left-stamps): New test.
(erc-scenarios-log--truncate): Move body to function of the same name,
and update assertions.
(erc-scenarios-log--truncate/left-stamps): New test.  (Bug#72736)

(cherry picked from commit 51d5419fdc3805a95190f8913e8ea349f243f11d)