feat(engine): #422 slice 1 — typed event vocabulary + EngineSink inversion for agent_stream

[justrach]

Slice 1 of #422: the engine stops writing to the terminal and starts emitting typed events through a strict sink interface. First offender converted: agent_stream.zig now has zero imports of term.zig/agent_render.zig/ansi.zig/anim.zig.

What's in it

• engine_events.zig — the new internal vocabulary: one tagged union covering every emission agent_stream produces (stream_begin, reasoning_delta, text_delta, thinking_fold_toggle, stream_aborted{interrupted|stalled|dropped}, stream_complete, stream_finished) plus Cursor{generation, sequence}.
• protocol_seq.writeEventStamped — cursor stamping integrates with the existing seq counter (durable events reserve ids inside the stdout lock; pulses observe) so the --json wire's gap-free numbering and bytes are unchanged. One counter, no duplication.
• engine_sink.zig — strict EngineSink vtable (sinks receive events only). TuiSink reproduces every old inline TTY branch gate-for-gate via agent_stream_render.zig (spinner + live Thinking block moved verbatim); JsonSink emits the existing wire lines byte-identically, pinned by test.
• agent_output.zig — say/sayApiError/emit moved out of agent.zig (now 538/600) with member aliases so call sites are untouched.

Evidence it changes nothing

A before/after eval harness (goldens captured from v0.0.239 main, determinism proven by double-run) gates the change:

• --json protocol: scripted session against a local mock Codex backend (tool call + two text turns + mode/effort controls) — full 22-event NDJSON stream, stderr, exit code, and backend request count byte-identical to baseline.
• Interactive TUI: the same conversation on a real 40x120 PTY — 79-line normalized transcript byte-identical to baseline.
• zig build test: 981 → 989, delta exactly the 8 new unit tests (engine_events ×3, engine_sink ×4, protocol_seq ×1), all wired through test_hooks.zig; none removed. zig fmt --check clean.

Known slice-1 debt (documented in-code, on the epic's work list)

• agent_ws.zig still reaches spinner state via Agent aliases; agent_argstream.emitArgText still branches json/tty inline — next offenders.
• TuiSink wraps *Agent as its render-state handle until the render state moves out.
• Cursor.bumpGeneration exists but nothing calls it yet — lands with the serve/attach slice (Local daemon: graff agents / graff attach — sessions that survive terminal close #420).
• Per-stream md/thinking resets stay inline deliberately: they ran unconditionally in all modes before, and behavior preservation beats purity in this slice.

Part of #422.