feat(center-stage): multi-person group framing + shot-size headroom + lead-room

[TCVinNYC]

What & why

Adds the marquee Center-Stage feature AutoPTZ lacked — multi-person framing — plus two composition-polish knobs, all in the digital framer path (no physical-PTZ controller changes). Every default reproduces prior behaviour exactly.

D1 — Multi-person group framing (union bbox)

• New tracking.group_framing: bool = False toggle (off → no behaviour change).
• union_bbox() pure helper in digital_framer.py + CameraWorker._group_union_bbox() over the live confident, non-lost TrackInfo boxes.
• desired_crop now sizes the crop to fit the subject both vertically and horizontally, so a WIDE union auto-widens (zoom out) to keep everyone in shot — still aspect-locked and capped by max_frac. Tall single-person boxes stay height-driven exactly as before.
• Group-vs-explicit-lock rule: an explicitly locked target (by track id OR configured identity) always wins — Center Stage keeps following that one person even with group mode on. Group framing applies only when no explicit target is locked. The existing trusted_bbox fallback for single-target churn is preserved.

D2 — Shot-size-aware headroom

• _CENTERSTAGE_FRAMING carries a per-preset headroom: face 0.06 → head_shoulders 0.08 → upper_body 0.10 (unchanged midpoint) → full_body 0.14. Closer shots get less margin above the head, full-body more.
• _framed_output sets DigitalFramer.headroom from the active preset, the same way fill/max_frac are already wired (live, no restart).

D2 — Subtle digital lead-room ("nose room")

• New tracking.lead_room: float = 0.0 gain (default off / centred, lead=0 reproduces prior framing).
• DigitalFramer tracks an EMA of the framed subject's centre velocity and offsets the crop centre toward the motion, capped to 12% of the crop so it can't destabilise framing.

Tests

• union_bbox / _group_union_bbox (pure) tested directly; group crop frames the UNION and is wider than any single box; lost people ignored.
• Explicit-lock-wins rule tested (locked id and locked identity-via-trusted-bbox both beat the group union).
• Headroom-by-preset: more headroom lifts the crop; framer field is applied.
• Lead-room: subject moving right shifts the crop centre right; lead=0 → centred as before; default stays subtle.
• All existing digital_framer tests (dead-zone hold, aspect lock, split size smoothing) preserved.

Gates

• ruff check / ruff format --check: clean
• mypy autoptz/engine/runtime/ autoptz/config/ (strict): clean
• pytest tests/: 1325 passed
• python -m autoptz --selftest: all checks passed

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