build: modernize Python support and decouple streaming env from PsychoPy & VR deps

[pellet]

build / CI:

• CI matrix expanded — default Python 3.10 (was 3.8); experimental jobs on py3.11 (full env), py3.12 / py3.13 (streaming env).
• dropped support for Python 3.8 and 3.9 across all conda envs and requirements.
• matrix now parameterized by env_file / env_name so streaming-only builds can run without psychopy/psychxr.
• typecheck job moved from py3.9 to py3.10.

dependencies:

• psychopy bumped to fork pellet/psychopy@v2026.2.0-rift-fix — fixes Rift stereo projection-matrix crash under strict-ndim psychxr.
• psychopy-sounddevice switched from local editable install to official upstream Git tip (handles macOS arm64 sound).
• psychxr — prebuilt Windows wheels from pellet/psychxr fork for cp310–cp313 (PyPI only ships 0.2.4 for ≤ py3.9). Adds experimental Quest-link VR support on py3.10–3.13.
• pyobjc bumped 7.3 → ≥8.0 for newer psychopy.
• pyxid2 added to streaming requirements.
• pywinhook removed — obsolete once py3.9 was dropped (modern pynput on py3.10+ doesn't need it).
• pyo completely removed from dependencies (audio was never needed for Analysis/Streaming, and fails to build on py3.11).
• numpy py3.8 pin removed (numpy>=1.26 across the board).
• setuptools<81 pinned in docsbuild reqs — brainflow imports pkg_resources at runtime, which setuptools 81 deprecated and 82 removed.
• test deps (pytest, pytest-cov, nbval) moved into streaming requirements so streaming-only CI can run them.

streaming env decoupling:

• new eegnb/devices/vr.py with class VR — encapsulates psychxr/Rift integration (clock sync, per-trial telemetry buffering, telemetry CSV save, optical-axis offset computation) behind a generic VR-device name. Imported lazily so the streaming-only conda env (no psychxr) can still use the package.
• lazy/optional imports added in eegnb/cli/utils.py, eegnb/cli/introprompt.py, eegnb/experiments/init.py so the package can be imported under a streaming-only env (no VR / no sound libs).
• new tests/test_acquisition.py (acquisition smoke test) — what the streaming-env CI job actually runs.
• new conftest.py for shared pytest setup.
• eegnb/devices/eeg.py — import pyxid2 wrapped in try/except so users without a Cedrus FTDI driver don't crash at import time.

experiment runtime:

• per-eye stimulus alignment now queried from the HMD runtime instead of being hard-coded to ±0.2. Quest 2/3 lenses are angled inward and offset within their own image, so a single fixed value left the checkerboard slightly off-centre and looking tilted outward. The new compute_optical_axis_offsets() asks the runtime for the actual per-lens position so the stimulus sits where each eye is naturally looking.
• trial loop now runs at higher OS scheduling priority (psychopy core.rush) with Python's garbage collector paused, so the GC can't pause stimulus rendering mid-trial.
• HMD-clock to system-clock sync moved into a single VR.sync_vr_clock() call at the start of each run, so per-trial telemetry timestamps line up with EEG markers.