openapi: split schema lowering into one file per construct #1060

Problem

In compilers/openapi/internal/schema:

• compose.go (1,588 lines) holds four constructs in sequence: lowerAllOf at line 41, lowerOneOfAnyOf at 549, lowerDiscriminator at 1136 and lowerEnum at 1382.
• schema.go (2,420 lines) holds dispatch beside the keyword census, models, scalars, $dynamicRef and nullability.
• resolve.go lowers a $ref site, although resolving references is internal/resolve's job.
• discriminated.go and enummember.go hold halves of constructs whose other halves sit in compose.go.
• The naming hints for a hoisted node are split between resolve.go and compose.go, each calling the other.

allOf's tests are split too: 39 TestAllOf_* in compose_test.go, 16 in schema_test.go.

Direction

• Tests follow their names. foreign_test.go, mapping_raw_targets_test.go, unhomed_discriminator_test.go, reasons_internal_test.go and fix(compilers/openapi): flatten a nested inline allOf #998's compose_equivalence_test.go merge into the files of the constructs they test.
• Comments that name a file: FillPropertyDetail (schema.go) in internal/annotation; the compose.go path in compilers/openapi/amplification_test.go; and the note in accumulate.go about what operation reaches in schema.go, which is re-derived, not just re-pointed.

Blocked by

• fix(compilers/openapi): flatten a nested inline allOf #998 and fix(compilers/openapi): judge redeclared allOf field types #1000 edit compose.go, schema.go and their tests.
• feat(compilers/openapi)!: validate against an OpenAPI rule table #1046 edits schema.go and schema_test.go.
• The change for openapi: decide how nullable in an OpenAPI 3.1 document is treated #632 edits nullability in schema.go.
• openapi: split annotation.go by construct #1056 owns annotation.go, where one of the comments above lives.

Done when

• Every top-level declaration of the package, with its doc comment and any comment between declarations, is byte-identical before and after the move (compared with a throwaway go/ast script), and no golden changes.
• Comments that name a file name the file the code now lives in, changed in a commit of their own.
• go test ./compilers/openapi/... passes without -update.

Priority: P4, rule 1 (housekeeping: moves code between files and changes no lowering, validation, IR or emit-contract behaviour). wc -l compilers/openapi/internal/schema/{compose,schema}.go → 1588, 2420 (morphic @a1f15b29).