docs(readme): document Oura ring support — what works, what's coming, what can't

[pipiche38]

The README describes NOOP as a WHOOP-only project. The tree has had experimental Oura Ring (Gen 3)
support for some time — it pairs, offloads a night, and shows real heart rate, sleep stages, skin
temperature and motion — and a reader has no way to learn that from the front page. Someone with a ring
either assumes it's unsupported, or tries it and has no idea which of their expectations are reasonable.

Docs only. No code, no behaviour change, no new capability implied.

What it adds

• A row in the Strap support table.
• A new Oura ring support section, structured as what works / what's in progress / what can't
  work from this data.
• One line in the intro, and the same not-affiliated / clean-room note the WHOOP text carries.

The judgement calls, since this is the part worth reviewing

I wrote it deliberately conservative — over-promising on the front page is worse than saying nothing:

• Marked experimental and explicitly not a supported strap, matching how it's gated in the wizard.
• iOS/Android only, stated up front. macOS pairing is a reproducible silent hang (§3.8) and someone
  on the Mac reference app shouldn't lose an evening to it.
• "A ring-only day doesn't score yet" is called out as the biggest gap, not buried — it's the first
  thing a new ring owner notices.
• HRV / RMSSD / SDNN, Rhythm, and respiratory rate are marked ❌ "not possible from this data", not
  "coming soon". The ring banks intervals rather than sending beat-to-beat values; NOOP refuses a number
  there instead of showing a plausible wrong one, and the README should say that in the same voice.
• SpO₂ is described as a stored raw channel, not a Blood Oxygen reading, because the calibration
  question (~47% of samples decode above 100%) is still open.

On the Auth Key

The section says what it does, then bounds it, rather than calling it "unlocks everything":

• it inherits the account's cloud entitlements, so it needs an active paid Oura membership at the
  time each feature is enabled;
• what happens after a membership lapses is untested — one billing month is what was actually
  observed, and the text says that rather than implying permanence;
• it does not unlock the ❌ rows, since those are limited by what the interval stream physically
  contains, not by an entitlement.

Every claim traces to something already in-tree (OURA_PROTOCOL.md §3.7 / §3.8 / §7.1) or to
hardware-validated work already merged.

Not done, on purpose

• No header badge. The existing badge says "works with WHOOP 4.0 & 5.0"; adding Oura beside it would
  read as a support claim the ❌ rows contradict. Easy to add if you'd rather.
• The other experimental brands (Amazfit, Mi Band, Garmin) are still undocumented here. Oura is much
  further along, so this covers only Oura rather than pretending to a general experimental-device page.