feat(lifecycle): proposition lifecycle status, pinning, and decay

[jimador]

Gives a proposition a lifecycle so it can age and be retired. Status gains a STALE value, and a proposition can be pinned to exempt it from decay. The single revised timestamp splits into contentRevised (the decay anchor) and metadataRevised (administrative touches) so editing a proposition's text and re-touching its bookkeeping are no longer the same event; revised remains as a deprecated alias for contentRevised.

• StatusTransitionPolicy decides which status transitions are allowed
• DecaySweeper marks propositions stale as their confidence decays, leaving pinned ones alone

Existing revisers that stamped revised now stamp contentRevised.

Related issues

This is an incremental, self-contained slice; it advances these issues without fully closing the broader ones.

• Relates to Content-aware decay and revision for propositions #10 (Content-aware decay and revision) — adds the scheduled-sweep write-back (DecaySweeper) and policy-driven decay SPI (StatusTransitionPolicy / DecayStatusPolicy) the issue proposes.
• Relates to Proposition pinning and eviction immunity #9 (Proposition pinning and eviction immunity) — pinned propositions are decay-immune (the sweep skips them); full eviction/budget behavior is out of scope here.
• Relates to Proposition lifecycle events #8 (Proposition lifecycle events) — exercises status-change transitions over the existing lifecycle event surface.