feat(shm): add ZM 1.38 SharedData struct support with size-based auto-detection

[pliablepixels]

Summary

ZoneMinder 1.38 changed the SharedData struct layout in shared memory, adding new fields (image_count, latitude, longitude, capturing, analysing, recording, janus_pin, etc.) and renaming active → capturing. The struct size grew from 760 to 872 bytes.

Changes

• Replace inline Perl comment blocks with declarative _LAYOUT_760 and _LAYOUT_872 tuple lists
• Add _build_struct_info() helper to derive format string, field names, and string-field list from layouts
• Build a _REGISTRY dict keyed by struct.calcsize() for automatic version detection
• Auto-detect ZM version by reading the size field (first uint32) from shared memory and looking it up in the registry
• Extract TriggerData format to module-level constants
• Add backward-compatibility alias: active ← capturing for ZM 1.38

Design

Adding support for a future ZM version requires only:

1. Define a new _LAYOUT_xxx list
2. Append it to the registry loop

No if/else branches, no duplicate format strings.

Related: PR ZoneMinder#58 by @jantman