fix(mac): CGDisplayStream fallback when SCShareableContent misses virtual display

[hmachine1999]

Summary

Fixes the hard failure path reported in #142:

virtual display never appeared in SCShareableContent (MacSender Code=3)

On affected machines (reproduced: Mac mini M2 + single external 4K display + macOS 15.7.x):

1. Wi‑Fi connect + phone hello succeed
2. CGVirtualDisplay is created and returns an id
3. SCShareableContent never lists that id → capture never starts
4. No purple recording indicator; iPad stays on the OpenDisplay UI

This is not a network failure — enumeration fails after a successful handshake.

Changes

File	Change
Mac/VirtualDisplay.swift	waitUntilReady() — poll online/NSScreen after applySettings (async)
Mac/MacSender.swift	Richer findSCDisplay (explicit SCK API, size match, diagnostic logs)
Mac/MacSender.swift	CGDisplayStream fallback when SCK times out, using the known CGDirectDisplayID
Mac/MacSender.swift	Tear down displayStream in stop() / reconfigure()

ScreenCaptureKit remains the preferred path. CGDisplayStream is deprecated and used only as a last resort so users are not stuck with Code=3 when the virtual display id is already known.

Test plan

• Local Debug build succeeds (OpenSidecarMac)
• On a machine that previously hit Code=3 every session with stock builds: Extend over Wi‑Fi streams after this change (fallback path)
• Maintainer: Extend on a normal multi-display Mac still uses SCK (no fallback log line)
• Maintainer: Mirror mode unchanged
• Maintainer: Disconnect / reconnect / rotate still tear down capture cleanly
• Maintainer: /tmp/opensidecar-mac.log shows either normal capture started: or capture started (CGDisplayStream fallback):

Notes / non-goals

• Does not change stable per-device display serials or arrangement restore (product behavior kept).
• Optional follow-ups from Mac: virtual display created but never appears in SCShareableContent (Code=3) — no capture / purple indicator; CGDisplayStream fallback works #142 (FAQ, SwitchResX EDID override docs) left out of this PR to keep the diff surgical.

Closes #142