Modernize to v2.1.0: PulseExpress API, expanded examples, bootloader

[awhitchurch]

• Rename driver class Max32664 -> PulseExpress and types Max32664* -> PulseExpress*; old names kept via max32664.h compatibility shim. Rename sources to protocentral_pulse_express.{h,cpp}. SPDX headers throughout.
• Fix earlier-firmware regression: begin() no longer hard-fails on non-40.x hubs; version check is now a soft warning exposed via firmwareSupported().
• Fix raw PPG streaming data loss: drain FIFO with a short read delay, clamp the OpenView 16-bit scaling, and make the OpenView send non-blocking so a host stall cannot back up and overflow the sensor FIFO.
• Add public readStatus() hub-diagnostics accessor.
• Expand examples from 3 to 11 (01.Name convention): heart rate/SpO2, BPT calibration, BPT estimation, EEPROM persistence, multi-subject calibration, HRV, device info/diagnostics.
• Add MAX32664 bootloader API (pulse_express_bootloader.{h,cpp}), the FirmwareFlash example, and a host flashing script under extras/. The .msbl firmware image is non-redistributable and is gitignored.
• Update README, keywords.txt, library.properties (version 2.1.0), add CHANGELOG.md and Uno R4 upload scripts.