fix(steps): distinguish missing day rows

[bhelm]

Summary

• preserve the existing no-step-counter explanation for truly empty input
• emit an honest noRowsForDay trace when non-empty input contains no rows for the requested local day
• avoid device-absence and motion-estimation claims in that filtered-empty case
• add mirrored Swift and Android public-contract tests for empty, out-of-day, one-row, and normal-total behavior

The current production live path remains guarded by day-bounded inputs and a non-empty check; this fixes the public helper contract without changing production wiring.

Verification

• Swift RED: out-of-day case produced noRawCounter plus three misleading device/motion claims
• Kotlin unchanged RED control: focused test failed on the same contract
• Swift focused: passed
• Swift trace class: 14/14 passed
• Swift full StrandAnalytics: 1,496/1,496 passed
• Kotlin focused and trace class: passed
• Kotlin full :app:testFullDebugUnitTest: 4,091/4,091 passed
• independent frozen-delta review: no P0-P2 findings
• git diff --check: clean

Android execution was serial and bounded: JDK 17, no daemon, one worker, no parallel execution, Kotlin in-process, 1536 MiB heap. Temporary Linux-only Swift build gates were fully reverted before commit.

Tracks bhelm#85