Report dangling references to the user instead of silently discarding them

[TobiasKlanert]

Context

The layout mapper (toElkGraph in core/layout/compose-to-elk.ts) filters
edges using a knownIds guard: If an edge points to a node that
does not exist, it is silently discarded. This is intentional because ELK
would otherwise abort the entire layout run with JsonImportException: Referenced shape does not exist.

Side effect: A depends_on, networks, or volume entry that references something
undeclared produces an incomplete graph without any
indication. The user sees a missing edge and doesn’t know why.

Problem

The guard prevents a crash but suppresses the information. For a tool
whose stated MVP goal is “understandable error feedback instead of a crash”
(see MVP Doc §2.1, point 3), silent rejection is the wrong final step.

Context

This is not a layout issue, but a validation issue. The parser currently checks
structure and required fields, but not the referential integrity
between services, networks, and volumes.

Possible Approach

• Add semantic validation to the parser: For each dependsOn,
  networks, and named volume reference, check whether the target exists in the
  top-level declarations.
• Collect unresolvable references as ParseError (using the same two-level
  mechanism as for existing semantic errors), specifying the
  service, reference type, and missing name.
• Decision pending: hard error (graph is not drawn) vs.
  warning (graph is drawn, a note is displayed). For dangling
  references, a warning is likely more user-friendly—the
  graph remains visible, and the note explains the missing edge.
• The knownIds guard in the mapper remains in place as a safety net, even
  when the parser validates—a double safeguard against the ELK crash.

Acceptance Criteria

• A reference to a nonexistent service/network/volume generates
  a clear message (reason + affected name).
• The graph does not crash (the guard remains active).
• Unit tests cover one dangling case per reference type.

Dependencies

Affects the error feedback UI (Roadmap Phase 6). It makes sense to
consider both together.