[LeonardSEO]

Experimental, and I do not expect this one to be accepted: it turns on by default a value the project deliberately keeps behind a default-off toggle until cross-device evidence agrees. I am opening it so the choice is visible; feel free to close it.

What this PR does

• The SpO2 candidate display toggle defaults to on (an explicit off still wins). Apple: PuffinExperiment.spo2CandidateDisplayEnabled and the Settings @AppStorage default; Android: NoopPrefs.spo2CandidateDisplay and its flow.
• The caption "strap estimate (unverified)" becomes "strap reading" in the Blood Oxygen tile and readouts, in all locales.
• The Health screen hint no longer says the estimate is "off by default".
• Nothing about the value changes: it is still never written to spo2Pct and never scored.

Reasoning: the strap computes this value itself during sleep (R18 byte 82, measured in 30 s windows every 1,200 s; the firmware logs "generated a valid SPO2 during sleep"), per LeonardSEO/wearable5.0 FINDINGS §13. It has still not been compared with the official app's SpO2 for a captured night.

Type of change

• New feature

How it was tested

• VitalReadingsTableTests and VitalSourceResolutionTests updated for the new caption; the Android caption test reads the resource, which keeps its id.
• ./gradlew testFullDebugUnitTest: 6688 tests, all pass apart from the 2 RecoveryDriversTest half-tie cases that fail the same way on unmodified main.
• Tools/i18n_audit.py --ci, Tools/doc_comment_lint.py, docs/protocol-examples checks and Tools/parity_ledger.py --refresh-derived (derived snapshots included): pass.
• Tools unit tests, the parity acceptance suite and the R-R contract: pass apart from the 4 test_check_source_references cases (/var vs /private/var temp path on macOS) that fail the same way on unmodified main.
• Apple: the macOS app (Strand) builds with this change on my development branch; StrandTests and the iOS target were not run locally for this branch. app-build.yml covers both.

Checklist

• Swift package tests pass for any package I touched (none touched)
• Android unit tests pass if I touched android/ (apart from the 2 RecoveryDriversTest half-tie cases that fail the same way on unmodified main)
• No new build warnings introduced (Android: none; Apple warnings not checked per branch)
• UI changes use only StrandDesign / Palette / Metrics tokens
• No hardcoded hex frame bytes; protocol facts live in the schema / decoders
• Follows the conventions in docs/CONTRIBUTING.md
• I did not commit generated output (Strand.xcodeproj/) or any secrets/keystores

Related issues

Refs #103

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