Explicit source stages for Go readers

Cypher source is projected once before compilation. External callers pass cypher.RawSource; compiler consumers pass cypher.AcceptedSource for text already projected by the execution source owner. The constants alias the same canonical type exported by antlr.SourceStage and fabric.SourceStage.

The required final argument applies to ResultColumns, StatementResultColumns, StatementResultColumnsContext, StatementFacts, StatementFactsWith, QueryPermissionRequirements and RegisteredProceduresForCalls, ResultProjection and ParseKnowledgePolicyDDL. resultmetadata.FactsProvider and the rewriting header interface carry the same explicit stage. For example:

columns, err := executor.StatementResultColumnsContext(ctx, clientQuery, cypher.RawSource)
permissions := cypher.QueryPermissionRequirements(ctx, clientQuery, cypher.RawSource)
command, owned, err := cypher.ParseKnowledgePolicyDDL(ctx, clientQuery, cypher.RawSource)

AuthorizeReadOnlyQuery remains a raw-client-source boundary. HTTP and Bolt query syntax is unchanged. A generated literal backslash followed by u0062 must remain literal compiler text; passing it as raw source would apply an unwanted second Unicode projection. Source identity is never guessed from contents or context. Raw metadata spans and participant obligations refer to original client bytes; cached execution products keep accepted coordinates.

ParseKnowledgePolicyDDL also requires the original context as its first argument. Execution retains the source view through nested policy blocks so persisted WHEN predicates and ON ACCESS expressions keep their original spelling. Accepted standalone input without a retained original is emitted as safe raw source before storage, preserving once-only evaluation after native reopen.

ANTLR syntax readers use the same explicit stage on one API:

syntax, err := antlr.ParseSyntax(ctx, clientQuery, antlr.StatementEntry, false, antlr.RawSource)
compiled, err := antlr.ParseSyntax(ctx, acceptedText, antlr.FragmentEntry, false, antlr.AcceptedSource)

The separate ParseSyntaxView entry point is retired. A caller holding a source view checks view.Err and passes view.Text with antlr.AcceptedSource. The request source retains original spelling; syntax cache identity includes the explicit stage, entry and dialect. Cancellation remains synchronous through the owned ANTLR character/token stream interfaces; the third-party runtime is unchanged.

Dynamic USE selectors also retain each argument's original byte extent through antlr.UseReference.Arguments and fabric.UseClause.Arguments. Each UseArgument contains Text and a half-open Span: original[Span.Start:Span.End] == Text. The existing Args strings remain available to routing consumers. Obtain these products through ReadUseReference or fabric.ReadUseClause with the appropriate source stage; do not reconstruct argument positions by searching repeated text.