Graph evidence 6/8: card section — strandings (dead-code delta) #13

Part of the graph-evidence integration.

What

Run prism dead-code on base and head; the card reports the delta — symbols newly unreachable because of this PR:

### Strandings
⚠ This PR makes 3 symbols newly unreachable:
    util/legacyEncode (last caller removed here)
    ...
Caveats: reflection, DI, serialization hooks, and codegen call symbols invisibly — verify before deleting.

Rules

• Delta only. Whole-repo dead code is noise on a PR; newly-stranded-by-this-change is signal.
• The engine's caveats field is part of the answer — always rendered, verbatim.
• exportedUnreferenced newly-added entries render separately as informational (exported liveness is not statically decidable in-project).
• Precision-first stance carried over: the engine only lists symbols that are unreachable AND non-exported AND name-unreferenced in all code text; the card can state that deletion is compile-safe.

Acceptance

• shale-test-bed PR removing a function's last caller → ⚠ naming the newly stranded symbol; unrelated pre-existing dead code does NOT appear.
• Base/head runs share the actions/cache from Graph evidence 2/8: CI plumbing — prism install + .grove cache in shale-action #9 (two graphs, one cold cost).