v0.2.1

No library changes since v0.2.0 — eas/ and wbxml/ public API and
behavior are unchanged. This is a test- and infrastructure-hardening
release.

Testing:

• Unit-test coverage 87.9% → 99.3% across eas/ and wbxml/. Every
  documented error path (state-store failures, malformed server
  responses, protocol-status branches, parser edge cases) now has a
  regression test. Remaining uncovered code is genuinely defensive
  stdlib failure paths (xml.Marshal, gzip.Buffer write/close,
  http.NewRequestWithContext with hard-coded inputs, pkcs7 internal
  errors, net.DefaultResolver fallback).
• wbxml exercises every header-truncation, MBUInt32 overflow, and
  unexpected-token path in encode/decode.
• New shared test helpers (errStateStore, errSentinel, countingState)
  for state-store failure injection across packages.

testenv:

• Z-Push BackendCombined IMAP pins to address the known
  SmartReply/SmartForward Status=120, FolderDelete HTTP 500, and
  Ping cold-boot Status=7 edges (#3): IMAP_DEFAULT_CHARSET=UTF-8,
  IMAP_INLINE_FORWARD=true, IMAP_FOLDER_PREFIX=''.
• testenv/README gains a 'Known limitations' section that documents
  these edges and the workarounds the integration tests already
  encode.

CI:

• gotestsum wraps both unit and integration test runs and emits
  JUnit XML alongside the existing coverage profile; uploads feed
  Codecov Test Analytics (with caveats — see codecov/feedback#637).
• README embeds the Codecov coverage sunburst chart under a new
  'Coverage map' subsection.