Generator refactors: a BindingContext record and a UnionDispatchEmitter #204

What

• SchemaAliasValidator and CurationValidator pass growing parameter clumps; extract a shared
  BindingContext record.
• tools/OpenCode.Sdk.Tools/Generator/Emission/UnionEmitter.cs is 1,046 lines; extract its
  marker-table and converter-dispatch half into a UnionDispatchEmitter.
• Trigger-gated: the two remaining binding-locality extractions, a binder test fixture derived from
  real scenarios and error-union binding moved into its own binder, wait until a generator change
  needs them.

The proof for each is an unchanged generated output.

Why it waits for 1.1

Internal refactors with no generated or public change.

Evidence

• tools/OpenCode.Sdk.Tools/Generator/Binding/SchemaAliasValidator.cs,
  tools/OpenCode.Sdk.Tools/Generator/Binding/CurationValidator.cs,
  tools/OpenCode.Sdk.Tools/Generator/Emission/UnionEmitter.cs.
• docs/ROADMAP.md:165-166 (Open Questions: the remaining binding-locality extractions).

Done when

• The two extractions land with generate --verify byte-identical and the tool tests green.
• The binding-locality row lands with the generator change that needs it, or closes with a reason.

Depends on

• The generator slice of Vertical-slice layout migration (placement only) #173.