[Compiler] Support related/secondary spans in conflict diagnostics

[cssbruno]

Problem

Diagnostics carry exactly one span. Diagnostic and ValidationError both have
a single Span/Pos, so conflict diagnostics — duplicate routes, contract
route conflicts, duplicate endpoints — can only point at one location. They
cannot say "first declared here, also declared here," which is exactly the
information a user needs to resolve a conflict.

This is timely: recent M2 work hardened contract-route conflict validation, and
those messages would be materially clearer with a related location.

Relevant code:

• internal/lang/diagnostic.go: Diagnostic (single Pos/Range)
• internal/compiler/validate.go: ValidationError (single Span)
• internal/compiler/routes.go, route_bindings.go, validate_contract_refs.go:
  route / endpoint / contract conflict validators

Scope

Add a related-locations field (e.g. Related []source.NamedSpan) to the
diagnostic contract, thread the first-declaration span through the conflict
validators, and surface related locations in check --json and LSP
relatedInformation.

Acceptance Criteria

• Conflict diagnostics include the primary span plus at least one related span
  pointing at the other declaration.
• check --json adds related locations additively; existing shape stays
  backward compatible.
• LSP maps related locations to relatedInformation.
• Tests cover duplicate routes and contract route conflicts with two locations.

Verification

go test ./internal/compiler ./internal/lang ./internal/lsp

[cssbruno]

Implemented and merged in #300.