proposal 0002: validate warning for dead renames overlay entries (#19)

[adworzynski]

Draft proposal for #19.

Adds a non-blocking validate warning (renames.dangling) for each manifest renames entry that resolves to no applied rename — the missing overlay-consistency mirror to the existing policy.dangling (policies) and relationship.overlay_dangling (relationships) diagnostics. Core, read-only, additive; no exit-code or schema change.

Key design decisions recorded in the doc:

• Detection via provenance, not id presence — a live kind: rule rename stamps source.original_id on the materialized rule; an entry is dead iff (pack, from) is absent from that applied-set. Precise reading of "from matches no active rule."
• Warn on forward-looking entries too, for parity with policy.dangling — a mistyped pack is indistinguishable from a legitimately-staged forward-looking one, and the warning register keeps a green repo green either way. Tiered/suppressing alternatives recorded as forks.
• One additive model touch: RuleDef gains a source field (the model currently discards the provenance the store carries).

Forks considered: do-nothing, refuse-at-rename add, to-id heuristic, tiered forward-looking suppression, fold-into-#8.

Refs #19.