darepod: re-expose GetStoredVTXO for harness inspection

[Roasbeef]

Summary

Restore the harness-only GetStoredVTXO accessor on darepod.Server
that was introduced in #239 and then accidentally removed during the
policy-migration refactor in 35dc614 (darepod: wire policy through daemon and CLI). The docs in darepod/CLAUDE.md and darepod/AGENTS.md
still reference this method, so the removal left the API surface
doc-inconsistent.

Motivation

The server-side integration harness (harness/arkharness.go in the
darepo repo) needs to inspect a client daemon's persisted
partial-unroll state during itests. Without this accessor:

• reaching into the unexported vtxoStore field across package+repo
  boundaries requires reflection or a visibility change, both of which
  break the "harness does not poke private daemon state" invariant
  that the client CLAUDE.md explicitly calls out;
• widening vtxoStore to an exported field exports the entire
  *db.VTXOPersistenceStore surface area when only a single lookup is
  needed.

Change

Re-add the 15-line accessor that delegates straight to
s.vtxoStore.GetVTXO(ctx, outpoint). Same shape as the original in
PR #239. No other code touched.

Test plan

• Client go build ./darepod/... clean.
• Server-side itests against this client commit (in the
  pr223-followup branch of darepo) run green.

[gemini-code-assist]

Code Review

This pull request introduces the GetStoredVTXO method to the Server struct in darepod/server.go, which allows test harnesses to inspect locally stored VTXO descriptors. The review feedback suggests improving the implementation by removing unnecessary blank lines and avoiding string concatenation in the error message for better readability.

[gemini-code-assist]

The implementation of GetStoredVTXO contains unnecessary blank lines and string concatenation in the error message. For better readability and searchability of error messages, it's preferred to use a single string literal and follow the formatting conventions used in the rest of the file (e.g., no blank line at the start of a function).

func (s *Server) GetStoredVTXO(ctx context.Context,
	outpoint wire.OutPoint) (*vtxo.Descriptor, error) {
	if s.vtxoStore == nil {
		return nil, fmt.Errorf("client daemon VTXO store not initialized")
	}

	return s.vtxoStore.GetVTXO(ctx, outpoint)
}