feat(stream): headless claude -p transport behind a transport flag

The PTY transport stands in for a protocol that did not exist when it
was written: it pastes bracketed text into claude's TUI, guesses when
the Ink render loop has settled, re-presses Enter when the paste is
swallowed, and tails the session JSONL at 100ms. `claude -p` with
stream-json on both pipes is that protocol, so add it as a second
transport and let callers pick with `BackendOptions.transport`.

Measured on one host, same model and prompt, cold single-reply turn:
pty burns 2.3s on TUI readiness before the prompt is even submitted
(first event 2.5s, turn 5.2s); stream_json burns none (first event
0.3s, first token 1.9s, turn 2.7s). The gap is what `startup_delay`'s
60s cap exists to survive on slow hardware.

Everything the old transport relies on carries over unchanged and was
verified against a real CLI: multi-turn over one live process, MCP via
--mcp-config, --resume, and `native_jsonl` history seeding. Two things
are new rather than equal: `result` is native (so usage is the turn's
aggregate and durations are real, not synthesized), and
`include_partial_messages` yields token-level `StreamEvent`s — which
the JSONL, holding only finished blocks, could never provide.

Notably, assistant records carry `stop_reason: null` in this mode even
at the end of a turn, so `result` is not just tidier than the PTY
path's terminal-stop_reason heuristic, it is the only correct signal.

Default stays `pty`; nothing changes for existing callers.

Also: extract process-group bookkeeping into `procgroup` so both
transports sweep claude's children on shutdown, and fix a stale
assertion in test_backend that still expected the `is_error: None` that
`_block_to_dict` deliberately stopped emitting.

tests/test_backend.py

@@ -836,7 +836,9 @@ def test_synthesize_turn_messages_covers_whole_cycle() -> None:
                     "type": "tool_result",
                     "tool_use_id": "tu_1",
                     "content": "result body",
-                    "is_error": None,
+                    # `is_error` is omitted rather than nulled — the
+                    # Anthropic API rejects a null with "Input should be
+                    # a valid boolean". See `_block_to_dict`.
                 }
             ],
         },