Adversary-dispatch preflight tuple can embed an inaccurate symbol path

## Summary
The orchestrator's SHA-preflight step (PG-9, ref #368) embeds a verified `{repo_root, branch, HEAD_SHA, specs_path}` tuple plus symbol locations in each adversary dispatch. There is currently no validation that the cited symbol PATHS resolve. A cited path can be wrong (e.g., a symbol placed in a plausible-but-incorrect file) even though the SHA is correct.

## Proposed remediation
The preflight helper should validate that each cited symbol path resolves in the tree at the pinned SHA (e.g., the symbol is grep-findable at the named path) before the dispatch is emitted, and warn otherwise.

## Impact
An inaccurate path cite is low-severity when the adversary re-derives locations itself, but it can send a fresh-context reviewer to the wrong file and waste a pass. Cheap to validate at preflight time.