Multi-screen M0: populate the per-monitor display registry on Linux (every plug-in's claims, displayxr-cli displays) #1850

Milestone M0 of the multi-screen plan (docs/roadmap/multi-screen.md, ADR-047 — PR #1849). Epic: #69.

Deliverable

displayxr-cli displays --claims on a Linux box lists every monitor with geometry, physical size, EDID ids, resolved plug-in and confidence. On ds1-linux: eDP-1 → sim-display (FALLBACK), HDMI-1 (Acer DS1) → leia-sr.

Runtime work

• Real Linux os_display_edid_enumerate: RandR monitors (os_display_desktop_enumerate) joined with sysfs DRM connectors (/sys/class/drm/card*-*/edid: manufacturer/product, serial, mm, connector name); join by output name, else mm, else pixel mode; DRM-only records with unknown origin on pure Wayland. Replaces the stub in os_display_edid_stubs.c that keeps build_dp_registry() a no-op off Windows.
• POSIX ensure_display_sources loads every manifest plug-in for probe_displays claims (mirrors Windows collect_display_sources_platform), honouring DXR_PLUGIN_EXCLUSIVE / XRT_PREFERRED_PLUGIN_ID. Active-plug-in selection unchanged.
• Hardware-free unit tests for the EDID parser and the join.

Plug-in work (displayxr-leia-plugin)

• Linux probe_displays (today NULL): list-returning EDID probe keyed by connector, claims at EDID/VERIFIED confidence, serial from the FPC (srLensGetSerialNumber on 1.38; SrDisplayDescriptor via srEnumerateDisplays on the LeiaSR Linux line ≥ 876620d62, compiled in when the headers have it). Refs Vendor SR-SDK requirements for multi-display routing (R1-R4) displayxr-leia-plugin#43, refactor: push eye-pos polling inside Leia DP via SR EyePairStream listener #251.

Out of scope

Hotplug, per-screen preferred plug-in, any compositor/oxr change, plug-in ABI changes (those are M1–M3).

Gate

CLI output on ds1-linux pasted in the PR; build_linux.sh + selftest green; Control Panel #793 phase 1 can read the same table afterwards.