state: add State presence and clear

[suraciii]

Summary

• add State.Exists and State.Clear
• keep absent State separate from a present Go zero value
• persist only present named State values
• clear one value while preserving other values and the Grain ETag
• keep conflict, unknown write, and activation discard behavior unchanged
• add Memory and SQLite tests for restart, zero values, clear, conflicts, and failures

Verification

• focused State tests passed
• make test passed
• make sim passed
• make ci passed
• diff check passed

Closes #92