[Compiler] Extend related spans to remaining duplicate/conflict diagnostics

[cssbruno]

Follow-up from #297 and PR #300.

Problem

#297 added related "first declared here" locations to duplicate_route and
route_method_conflict (including contract-route conflicts). The diagnostic
contract now supports Related []source.RelatedSpan, but the rest of the
duplicate/conflict diagnostics still point at only the second occurrence and
leave the first declaration unlinked.

Relevant code (each builds a ValidationError without Related):

• internal/compiler/validate_identity.go: duplicate_page_id,
  duplicate_layout_id, duplicate_component_name, duplicate_component_emit
• internal/compiler/validate_page.go: duplicate_revalidate_policy,
  duplicate_page_store, duplicate_css_selection
• internal/compiler/validate_source_uses.go: duplicate_gowdk_use_alias
• internal/compiler/validate_component_contracts.go: duplicate_go_import_alias
• internal/compiler/routes.go: duplicate_route_param
• Go endpoint comment duplicates: duplicate_go_endpoint_comment

Scope

Thread the first-declaration span into the remaining duplicate/conflict
diagnostics using the existing relatedSpan helper (or an equivalent), so each
reports the primary span plus a related location. Where the first span is
genuinely unavailable, leave Related nil rather than fabricate a location.

Acceptance Criteria

• Each listed duplicate/conflict diagnostic carries a related location pointing
  at the first declaration when its span is known.
• check --json related array and LSP relatedInformation populate for them.
• Tests cover at least the id, store, use-alias, and route-param duplicates with
  two locations.

Verification

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

[cssbruno]

Implemented and merged in #307.