Support per-unit lifecycle callbacks for library services

[JaxCalder]

Support paired optional per-unit START/STOP DIF callbacks for type=library services, passing expanded arguments. Track unit activity independently of module enablement; stopping one unit keeps shared API consumers available. Plain libraries retain existing behavior.

Enable before DIF resolution for cold startup. Refresh inactive template arguments on a new device event so %v cannot retain an old path. Publish the generic contract in libsystemd_service.h.

Validation: libsystemd 62/62; systemd/service frontends 1/1 each; full Cortex-M7 build. STM32F746G-DISCO automatic library service startup after reset: UTC 6/6, RTC 12/12, RNG 6/6. Tests cover cold load separately from prior API enable, multiple units, failed starts, removal/re-add and changed paths.

Consumer: choco-technologies/dmutc#1. Release libsystemd with this header before building/releasing the consumer from public packages.

Related implementation PRs:

• RTC reporting: Report RTC devices to libsystemd for device rules dmdevfs#73
• Generic library service lifecycle: Support per-unit lifecycle callbacks for library services #8
• UTC library: Implement UTC library with rule-driven RTC services dmutc#1
• Boot configuration: Install rule-driven UTC library service configuration dmod-boot#132
• Ecosystem: Integrate RTC reporting and rule-driven UTC library services dmod-ecosystem#31
• Registry: Register dmutc UTC library dmod-registry#17

Additional dmboot integration regression: the existing CI Renode runner passed boot/log-ring access, UART/VFS, SD automount and persistence across remount, LCD and touch tests. A second run with the optional audio model also passed. A one-second 523 Hz tone completed with zero underruns; captured stereo channels agree and measure approximately 522 Hz.

The Renode network/TAP step was explicitly skipped because this host lacks root and a TUN device. Physical STM32F746G-DISCO checks passed DHCP, ping in both directions, telnet command execution and temporary root/SD file read/write. The ICMP library correctly rejects STOP while a dependent module uses it. The independent plain-library lifecycle regression passes; libsystemd now passes 63/63. No production code fixes were needed in this regression run. Raw validation logs remain local and are not published.

Follow-up network diagnosis: a single ping after idle returns 0/1; an immediate repeat returns 1/1. An outgoing board ping warming ARP is followed by 3/3 incoming responses. Released ICMP code sends replies inline on the sole RX pump, while its send path can block waiting for an ARP reply that this same pump must consume. This explains the cold-cache loss and predates these UTC/library-service changes. No networking implementation was changed. Network regression therefore remains qualified rather than declared loss-free.

The libsystemd GitHub CI passes with the 63-step suite. Raw logs remain local. Two small physical-board test files remain because rm is absent; installing an additional removal tool was rejected by automatic approval review. No extra executable was installed on the board.