feat(engine): canonical types & value boundary

[sepehr-safari]

What & why

First slice of S1 — Engine core: the pure-Zig OCPP engine's foundational
data model under src/ocpp/. This is the headless, UI-free contract every later
engine module (normalizer, parser, timeline, detection) builds on.

It mirrors the toolkit's core/types.ts — the shared conformance contract —
rather than reimplementing its prose: the shapes and enums match so Studio can
later prove byte-for-byte parity against the same fixtures.

Changes

• src/ocpp/types.zig — Direction / MessageType / Status enums (with
  wire-string ⇄ value and MessageTypeId conversions), plus Event,
  TraceEventInput, Trace (+ TraceMetadata), ParseWarning, ParseResult,
  and Session.
• Value boundary (ADR-0005) — arbitrary OCPP payloads and raw message arrays
  are carried as std.json.Value over a single per-parse arena, behind a
  version-tagged decode seam so OCPP 2.0.1 is additive, not a rewrite.
• src/ocpp/ocpp.zig — engine aggregate root; test discovery wired into the
  root test block so native test exercises engine tests.
• Docs — ADR-0005 + index entry; CURRENT_STATE.md and AGENTS.md updated
  in-PR (S1 now in progress; src/ocpp/ added to the tree).

Scope boundaries

Detection types (Failure / FailureCode / FailureSeverity), SessionSummary,
and scenario/assertion types are intentionally deferred to the milestone that
produces them (detection is S2). S1 defines only the parse → normalize →
correlate model.

Testing

• native test -Dplatform=null → 8/8 pass (4 new engine-type tests:
  enum encodings, and a parsed-message-through-Event end-to-end model test).
• native check --strict → clean.
• zig fmt --check → clean.
• Engine modules import only std — no native_sdk / runner (verified pure).

Closes #11