v0.3.0

Functional-bug, correctness, and SDLC fixes from a full code review. Two more masked
Zig-0.16 build breaks (same class as the 0.2.2 streaming fix) were found and fixed.

Breaking

• Removed the connection-pool API (ConnectionPool, PoolManager, PoolStats). It
  was never wired into K8sClient, leaked connections, and std.http.Client already
  pools internally. (H2)

Fixed

• JSON field binding: List(T).metadata.continue and ServiceSpec/SecretData/
  deployment-strategy type (and WatchEvent.type) now bind the real wire names —
  Zig's std.json does no underscore stripping, so pagination tokens and Service
  type were silently dropped/mis-serialized. (H1)
• Retry actually runs: idempotent CRUD (GET/PUT/DELETE/PATCH) is retried per
  retry_config; previously every operation used the non-retrying path. POST stays
  single-attempt. (H3)
• apply/auth/crd JSON serialization didn't compile on 0.16 (used removed
  std.json.stringify, ArrayList.writer, and managed map/list APIs) — masked by
  lazy compilation. Migrated to std.json.Stringify.valueAlloc + unmanaged
  collections; StrategicMergePatch now stores a serializable ObjectMap.
• Plugged error-path memory leaks in the kubeconfig parse helpers (errdefer per
  duped field). Added ClusterInfo.deinit.

Security / hardening

• exec_credential logs via std.log instead of stderr; documented that
  ExecConfig.env is not yet applied. proxy_fallback now logs the TLS→plaintext
  downgrade instead of failing open silently. Documented that K8sClient is
  single-threaded (last_api_error is unsynchronized).

CI / DevEx

• build.zig.zon git-pins zig-yaml (was a relative path) so a clean clone builds;
  CI drops the sibling checkout.
• CI: zig fmt --check gate, {ubuntu, macos} build matrix, non-blocking Zig-master
  canary. Release attaches SHA256SUMS.
• Added SECURITY.md, CONTRIBUTING.md, CODEOWNERS, a PR template, a README
  Stability section; rewrote the stale docs/TESTING.md; removed dead
  websocket_live_test.zig; zig fmt across the tree.
• The migration probe now force-compiles every public method body (_ = &T.method),
  so removed-API breakage fails the build instead of a downstream consumer.