plan: add freebsd-notifyd porting plan

Companion to launchd/configd/kmodloader/asl. Ports Apple's notifyd +
libnotify (APSL, latest tag Libnotify-98.5 at apple-oss-distributions/
Libnotify) — the lightweight named-event pub/sub bus that ~everything
on macOS uses for cross-process coordination.

Drops Mach IPC + Mach-allocated shared memory. Replaces with:
GNUstep Distributed Objects over AF_UNIX for the daemon-client
channel; POSIX shm_open(2) + mmap(2) for the state-value pages
(preserves Apple's zero-IPC-read characteristic for high-frequency
state queries — volume level, network reachability, etc.).

Library (libnotify) C API stays byte-for-byte stable. Apple-derived
apps (gershwin desktop, third-party Cocoa ports) link our
libnotify.so unchanged. Source compat is mandatory.

Drops one delivery type: NOTIFY_TYPE_PORT (Mach port wakeup) — apps
source-port to NOTIFY_TYPE_DISPATCH or NOTIFY_TYPE_FD. Net code change
in consumer apps is ~5 lines.

Lives in the monorepo under notifyd/ alongside src/ (launchd),
configd/, kmodloader/, asl/.

Phase 7+ work — sequenced after configd lands. Promotion happens
when first Apple-derived app shows up needing libnotify, OR when
configd's DO-callback fanout becomes a many-subscribers bottleneck.

Significant section §8 details concrete gershwin use cases: theme
fanout, app activation events, battery/power state, network state,
NSUserDefaults cross-app coordination, and the launchd LaunchEvents
integration that replaces the proposed RequiresPhase mechanism with
a more general notifyd-driven model.

Author: pkgdemon