openapi: split load.go by concern #1058

Problem

The rest of compilers/openapi/internal/load keeps one concern per file (later.go, required.go, references.go). load.go (1,150 lines) still holds four concerns beside the load itself:

• the source budgets;
• YAML stream decoding;
• the parser's validation findings;
• overlay application.

Direction

• budgets.go: exceeds, budgetRefusal, OverByteBudget, nodeCount (it pairs with the existing budgets_internal_test.go).
• decode.go: the YAML stream decoding and Parsed.
• overlays.go: patch, applyOverlay, overlayRefusals.
• The parser's validation findings join validate.go, beside the rule-table validation feat(compilers/openapi)!: validate against an OpenAPI rule table #1046 adds under that name. If that file turns out to hold something else, the findings take a name that does not collide, and the PR says why.
• load.go keeps Load, its options, the resolution setup and the supported versions. The package comment moves to doc.go.
• Tests in load_internal_test.go follow what they call; the package's other test files stay as they are.

Blocked by

#1046 rewrites load.go, load_internal_test.go and most of the package, and adds validate.go.

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/load/load.go → 1150 (morphic @a1f15b29).