RFC #623: cascade-specific lifecycle merge rules across foundation+platform layers

## Context

Open question from RFC #623 (Token Lifecycle Metadata), tracked in `docs/rfc-coordination.md`.

When a token exists in both a foundation layer and a platform layer with different `lifecycle` values (e.g. foundation marks it `deprecated`, platform overrides with `stable`), define the merge semantics: which layer wins, whether platform can un-deprecate a foundation token, and whether the resolver should emit a diagnostic when lifecycles conflict.

## References
- RFC #623: https://github.com/adobe/spectrum-design-data/discussions/623
- `docs/rfc-coordination.md` RFC #623 row
- `spec/token-format.md` (lifecycle table)

Start by reading RFC #623 in the linked discussion, then review the RFC #623 row in docs/rfc-coordination.md and the lifecycle table in spec/token-format.md. The work is complete when the foundation/platform precedence, un-deprecation behavior, and conflict-diagnostic decision are agreed and documented in the referenced materials.

Written by the indexing model from the issue text.