fix: free the MonitorLink each linked-monitor token creates

[nabbi]

Automated PR, tested in a zm-holodeck sandbox. An AI agent (Claude Opus 5.5) reproduced
the bug on a disposable ZoneMinder built from upstream master c3d2e542b with
simulated cameras, wrote this fix and ran the checks below on the same lab. Minimal
human review before opening: please review it as you would any contributor's PR.

TL;DR

• Fixes: zmc leaks a file descriptor and a shared memory mapping per linked monitor on every reconnect or reload #5218: #: zmc leaks a MonitorLink (with an fd and a shared memory mapping once used) per linked monitor on every reconnect or SIGHUP reload.
• Change: Token holds its MonitorLink in a std::shared_ptr, so the token copies share it and the last one frees it with the expression tree; 1 file, +6/-5
• Risk: linked-monitor alarms go through the same objects; the events check's linked scenario (motion on the source records a "Linked" event on the target) passed on the fixed build
• Tested: reproduced before (+1 fd and mapping per reload), flat after (1 fd, 1 mapping over 10 reloads); checks covering the diff green

Details: reproduction, change, checks

Before

Upstream master c3d2e542b, clean plain build; monitor 2 linked to monitor 1, ten SIGHUP
reloads of zmc -m 2 (zmdc.pl reload), 8 s apart:

round=0 pid=36847 fds_on_zm.mmap.1=1 maps_of_zm.mmap.1=1 open_fds=11 rss_kB=150216
round=1 pid=36847 fds_on_zm.mmap.1=2 maps_of_zm.mmap.1=2 open_fds=12 rss_kB=156824
...
round=10 pid=36847 fds_on_zm.mmap.1=11 maps_of_zm.mmap.1=11 open_fds=21 rss_kB=156620

Change

• src/zm_monitorlink_token.h: monitor_link_ becomes std::shared_ptr<Monitor::MonitorLink>,
  made with std::make_shared. Tokens are copied (parse() copies them into the tree), which is
  why a delete in ~Token alone would double-free; a shared owner fits that. The defaulted
  copy and move constructors lose constexpr (not allowed with a shared_ptr member in C++17).
• Test: no unit test: a monitor-link token loads a monitor from the database
  (Monitor::Load), which the unit tests don't have. Covered by the probe above and the events
  check's linked scenario.

Behaviour changes

Change	Who sees it	Intended / collateral	Alternatives looked into	Chosen, why	Tested by
The MonitorLink, its Monitor, fd and mapping are freed when the expression is replaced (each connect) or deleted (disconnect)	zmc with linked monitors	intended	delete in ~Token (double free: tokens are copied); unique_ptr (needs the tree to move tokens instead of copying them, a larger change to the parser)	shared_ptr	probe: fds/maps flat over 10 reloads; events linked PASS
Linking behaviour (when a linked monitor's alarm triggers)	users of Linked Monitors	no visible change			events linked PASS

Side fixes in this PR: none.

After

Branch fix/b77-monitorlink-token-leak at 91526d2e9 (base c3d2e542b), tested in a merge with
two other fixes from the same lab session (combo = master + this + fix/b55-usr-level-signal-safe

• fix/b65-zmc-stop-during-startup; no shared files with this one):

"head": "69fa500a6683a16c937ae8be4d249a7eb52ba861", "branch": "combo", "dirty": false (plain)
round=0 pid=373 fds_on_zm.mmap.1=1 maps_of_zm.mmap.1=1 open_fds=11 rss_kB=151824
round=1 pid=373 fds_on_zm.mmap.1=1 maps_of_zm.mmap.1=1 open_fds=11 rss_kB=190292
...
round=10 pid=373 fds_on_zm.mmap.1=1 maps_of_zm.mmap.1=1 open_fds=11 rss_kB=159196

Check	Result
holodeck checks for the diff (check-changed.py: unit-tests, script-tests, smoke, resilience, events, dblog --quick, signals --quick)	unit-tests ok (14725 assertions in 251 test cases), script-tests PASS=31, smoke PASS=11 XFAIL=1 (stock zmonvif-probe.pl discovery, known), resilience PASS=5, events PASS=10 (record, mocord, modect-only, nodect, monitor, linked, jpegs, reload, filters, purge), dblog --quick PASS=1 XFAIL=2 (thread-error, contention: known on master, #5197/#5208), signals --quick PASS=4 (hup-default, usr, usr-threads, startup-stop)
pyzmNg / ha-zoneminder / zmNinjaNg	not run: no API, web or stream change
contract baselines (master)	not run: no PHP change

Not run: the deep set. Backport to release-1.38: same code, the commit cherry-picks cleanly (not built).

Tested as 7cde4e059; 91526d2e9 is the same tree with fixes #5218 added to the message.

fixes #5218

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