Add onboarding & gesture docs, sensor field notes, and improve optical sensor normalization

[bseverns]

Motivation

• Provide a concise, teachable onboarding path so new collaborators can run and teach the full pipeline in a single session.
• Make the gesture heuristics narratable and tied to firmware knobs so instructors can tune and explain behavior live.
• Expand sensor front‑ends from terse stubs into readable field notes that are classroom‑friendly and actionable.
• Improve the optical front‑end so the normalization and baseline behavior is robust and narratable in noisy/low‑contrast rooms.

Description

• Add docs/Onboarding90Min.md with a 90‑minute workshop syllabus that walks hardware → firmware → visualizer.
• Add docs/GestureVocabulary.md linking human‑readable gesture rules to the exact knobs in firmware/include/gesture_engine.h.
• Add sensor field notes docs/Sensors/OpticalReflective.md and docs/Sensors/CapacitiveSingleWire.md, plus update docs/Sensors/README.md, docs/SensingSurvey.md, and top‑level README.md to include the new pages.
• Enhance firmware/src/optical_sensor.cpp to track an ambient floor, subtract it before normalization, apply a small gain, and smooth the resulting envelope (ambient_floor_, gain_, env_), making the front‑end more stable and easier to teach.

Testing

• No automated tests were run on these changes.
• Changes are primarily documentation plus a small sensor front‑end tweak; recommended automated checks are pio test -d firmware -e native for gesture unit tests and building the firmware targets for CI validation.

---

Codex Task