Body lifecycle: zero-length chunks in streamBody, multipart boundary quoting, withResponseLogging after close() #77

Part of the audit remediation umbrella #67. Milestone 4. Severity: HIGH (first item), MEDIUM (rest). Requirements: BODY-10, HTTP-39, HTTP-51, BODY-26, BODY-28, IO-16.

Current SDK behavior

• Empty chunks reach the sink. #writeExactly (packages/core/src/body/stream-body.ts:79-91)
  reads, checks done, checks over-run, then writes. There is no value.length === 0 guard.
  A source that enqueues new Uint8Array(0) is never diagnosed; the auditor's probe forwarded
  200,000 empty chunks before the source closed, then reported
  EndOfStreamError: delivered 0 of 3 bytes. Each empty chunk reaches the transport sink.
  io/buffered-sink.ts:66-71 documents that a zero-length chunk is HTTP/1.1 chunked encoding's
  terminating chunk. io/retention-window.ts:177-183 and body/response-body-logging.ts:82-88
  enforce the rule that this path omits.
• Multipart boundary is not quoted. multipart-body.ts:160 builds
  multipart/form-data; boundary=${boundary}. BOUNDARY_PATTERN (:29-31) admits RFC 2046
  bchars, which include space, ,, :, =, ?, /, (, ). Those are not tchar.
  multipartBody(parts, 'a,b') yields a header Node's own FormData parser rejects with
  TypeError: Failed to parse body as FormData.
• snapshot() after close() poisons error(). withResponseLogging's snapshot()
  (response-body-logging.ts:241) calls startDrain unconditionally. closeDelegate (:62)
  already released the reader, so the drain throws
  TypeError: Invalid state: The reader is not attached to a stream. The catch at :119-122
  caches it, and error() (:244) returns it forever. read() after close() rejects with the
  same raw TypeError instead of ClosedResourceError.
• No Node-conformance coverage for the stream bridges. toReadableStream
  (io/buffered-source.ts:306), toWritableStream (io/buffered-sink.ts:127),
  io/tee-sink.ts:138, withRequestLogging, withResponseLogging have zero cases under
  tests/node-conformance/.

Expected behavior

streamBody treats a zero-length read for a positive request as a stream-contract violation
and writes no empty chunk to the sink. The multipart Content-Type a peer receives parses
with any RFC 9110 parameter parser. After close(), snapshot() returns the captured prefix
without starting a drain, read() rejects with ClosedResourceError, and error() reports
only a real drain failure. The pull, cancel, and lock behavior of the Web Streams bridges is
proven on Node as well as on Bun.

Notes and leads

• Empty chunks: if (value.length === 0) throw new SourceContractViolationError(...), same
  wording as retention-window.ts. Test an empty-only source and an empty chunk between real
  chunks.
• Boundary: quote when the boundary is not a pure tchar token (boundary="a,b"), reusing
  MediaType's parameter quoting. Or narrow validateBoundary to tchar and record the
  narrowing. Round-trip test through Node's Response.formData().
• Logging tap: check state.closed at the top of startDrain, snapshot, and read.
  Tests: close-then-snapshot, close-then-read, close-then-error.
• Node conformance: add one *.test.mjs per bridge under tests/node-conformance/. Cases
  belong there, not only in bun run test (CLAUDE.md, runtime-divergent surface).
• Patch changeset for @dexpace/core.