Fix retained metadata contracts and bound snapshot refresh memory

[mot-yelraf]

Summary

Persisted configuration edits previously emitted only non-retained patches, leaving offline subscribers with stale metadata. Publish a retained meta/config companion, advertise saved calibration/display/Time/HomeAssistant values, and include physical switch configuration with live switch state. Refresh persisted metadata one serialized snapshot at a time after command replies drain, reducing allocation pressure on CircuitPython boards.

Document the Sensorius subscriber migration and broker/heap validation procedure. Firmware version: v0.26.263.3.

Related issue

No separate issue; this focused change corrects the retained metadata contract and refresh behavior observed during operator validation.

Verification

• ruff check .: passed.
• pytest -q: 893 passed.
• git diff --check: passed.
• scripts/nodus_mpy.sh --target pico2w --no-stage-libs: compiled 70 modules with CircuitPython 9.2.8.
• scripts/nodus_mpy.sh --target xesp32s3 --no-stage-libs: compiled 70 modules with CircuitPython 10.2.1.
• Operator confirmed verification before requesting commit/publish/merge. Existing broker-visible Pico2 W observations and the acceptance procedure are recorded in docs/metadata_validation.md. The latest confirmation did not specify board/profile/duration/filesystem mode or attach a new capture; no additional hardware run was performed during PR publication. Full two-board acceptance coverage is not claimed.

Checklist

• I kept the change focused and avoided unrelated reformatting.
• I added or updated focused tests when behavior changed.
• ruff check . passes.
• pytest -q passes.
• I updated docs and TOML templates when public behavior or configuration changed (no new configuration keys).
• I did not commit credentials, live device configuration, private logs, or host-specific files.
• I preserved CircuitPython compatibility and constrained-memory behavior.
• I documented relevant hardware validation, or explained why it was not required.
• I updated the firmware version only if runtime behavior changed.