hal: restore working homecomp example

[grandixximo]

Follow-up to #4329. homecomp.comp was filtered out of the build because it no longer compiled on master; the discussion on #4329 made it clear that should be fixed. This restores homecomp as a real, buildable plug-in homing module that demonstrates something useful, rather than an empty "always-homed" shell.

The default skeleton is a working homing module:

• Creates the same per-joint hal pins the real homemod exposes: joint.N.home-sw-in, joint.N.homing, joint.N.homed, joint.N.home-state, joint.N.index-enable, for every joint seen at homing_init time. The output pins are driven once per servo period, so joint.N.homed tracks what motion actually believes.
• Adds a module-level pin motion.allow-sim-homed (default true). With it set, all joints report homed, which is useful for bring-up on hardware without working home switches. With it cleared, each joint reports homed only while its joint.N.home-sw-in pin is true, showing how HAL inputs wire into the homing API. The flag can be flipped from a postgui or M-code without rebuilding.
• Sizes its HAL buffer from n_joints at runtime (single hal_malloc), so the template does not depend on EMCMOT_MAX_JOINTS.
• do_homing() returns 1 on the not-allhomed => allhomed transition, matching base_do_homing() semantics so motmod switches FREE => teleop when homing completes.

The skeleton's function signatures use opaque pointer and integer arguments, so the default path does not pull in motion.h's heavy transitive includes. The exported symbols still resolve to the names motmod calls.

The advanced HOMING_BASE path is preserved: with HOMING_BASE pointing at a checkout's src/emc/motion/homing.c, the template #includes that file with CUSTOM_HOMEMODULE defined and exposes per-joint request-custom-homing / is-custom-homing pins for users who want to override the base_* helpers. This is the original pattern from the 2.9-era homecomp, kept for completeness. Note that motion.h/homing.h are no longer exported to the halcompile include path since 82f05bc, so the custom block relies on homing.c's own quoted includes and does not include them directly.

Verified: both build modes compile clean with halcompile (zero warnings), and the skeleton was runtime-tested headless as HOMEMOD=homecomp (sim on => all homed; sim off => unhomed; sim off with all home-sw-in closed => homed again).