feat(compilers/openapi): populate the operation streaming fields

[OmarAlJarrah]

Summary

Operation.Streaming, Operation.RequestStream and Operation.ResponseStream were written by no
compiler at any version. An SSE or NDJSON API declares what it does by naming a media type, and the
media type was copied to Content.MediaType and never read; a 3.2 itemSchema set Content.Item
and stopped there. Compiling either shape produced streaming: null with no diagnostic, so nothing
downstream could tell a stream from an ordinary body.

This adds the reading, in two halves that are deliberately not the same kind of fact:

• A 3.2 itemSchema declares the stream. The keyword states that the body is a sequence of
  items, so the operation streams and the declared item schema is the element type. Nothing is
  guessed, so no heuristic marker is stamped.
• A media type implies one. text/event-stream, application/x-ndjson and application/jsonl
  are read as frame formats through a new Options.StreamingMedia policy — default on, replaceable,
  and switchable off — with Provenance.Inferred naming the heuristic. That is architecture
  principle 6: the list is a default, not a standard, and a caller who wants only what a document
  declares turns it off. Matching ignores case and media-type parameters.

For a frame format the schema under the media type describes one frame rather than the whole body,
which is the opposite of how an ordinary content is read; StreamDetail.Events is what says so.
Where a direction offers several streaming contents the direction still streams but the element type
is left unnamed, with an info diagnostic: StreamDetail holds one Events per direction while the
payload keeps every media type, so electing one of them would be the primary-content selection a
compiler must not make.

Two notes on the issue as filed. Its acceptance item about an itemSchema "not double-applying"
assumed an existing 3.2 writer — there was none, so the gap was wider than reported rather than
narrower, and the two paths here cannot both fire because a declaration at a content wins over the
reading of its media type. And Provenance.Inferred holds a single string that path-prefix grouping
already used, so an operation reached by both heuristics now lists both rather than losing one.

Merge ordering: #345 also rewrites testdata/conformance/openapi/unwitnessed.golden.txt, and #341
adds DecodeOptions to the compiler contract. Whichever of the three lands first, the others rebase;
once #341 is in, StreamingMedia should gain textual option names there so the CLI's -opt path can
reach it.

Test plan

• New corpus rows streaming-media-30 and streaming-media-31 cover a 3.0 media-type-only stream, a
  bidirectional operation, a charset parameter on the media type, and the several-contents refusal.
• unwitnessed.golden.txt loses five entries — Operation.Streaming, Operation.RequestStream,
  Operation.ResponseStream, StreamDetail.Events and Provenance.Inferred, the last of which no
  corpus spec witnessed at all before.
• sequential-media now records the stream its itemSchema declares; its assertion pins that it is
  declared rather than inferred.
• A two-order test compiles the two streaming media types either way round and requires the streaming
  fields to agree, which is what an implementation electing the first candidate would fail.
• Policy tests cover the off switch, a caller's list replacing the defaults rather than extending
  them, and the exported default list being the one the lowering applies.
• Full gate: gofmt, vet, golangci-lint, build, and the 100% coverage gate.

Closes #250