[P2] Recovery HR windows document an inclusive end but drop aligned endpoint samples #39

Summary

restingHR documents and initially filters [start, end], but both it and recoveryIndexSlope use half-open five-minute bins and stop before a bin beginning at end. A sample exactly at an aligned end passes the prefilter and then belongs to no bin.

This is shared Swift/Kotlin behavior, not platform drift.

Minimal reproduction

Resting HR:

start = 0
end = 300
five samples at ts=300, bpm=60

Recovery Index:

start = 0
end = 1800
samples at ts=0,300,600,900,1200,1800

Observed behavior

On both platforms:

• restingHR returns nil/null; none of the admitted endpoint samples reaches a bin.
• recoveryIndexSlope returns nil/null; only five bins remain, below the six-bin gate.

Cause

The inclusive prefilter is combined with bins shaped as [t, t+300) and a loop that runs only while t < end.

Impact

An endpoint sample is silently admitted and then ignored. For Recovery Index this can change whether the minimum-bin gate is satisfied.

Acceptance criteria

• Define either [start, end) everywhere or handle an inclusive final boundary.
• Make documentation, prefiltering and bin membership consistent.
• Add paired aligned/non-aligned endpoint tests, including the five/six-bin gate.
• Preserve identical Swift/Kotlin behavior.

Synthetic timestamp fixtures are sufficient; no hardware testing is required.