refactor(realism): move emailgen LLM/personas/prompt into shared library

Lift the format-agnostic pieces from decnet/orchestrator/emailgen/
into the new decnet/realism/ library so file-class content generation
(stage 3 of the realism migration) can reuse them. Email-specific
delivery (RFC 2822 EML, IMAP/POP3 spool, thread chains) stays in
orchestrator/.

Renames (history-preserving git mv):
  emailgen/personas.py     -> realism/personas.py
  emailgen/prompt.py       -> realism/prompts/email.py
  emailgen/global_pool.py  -> realism/personas_pool.py
  emailgen/llm/            -> realism/llm/

Env-var clean break (pre-v1, no aliases):
  DECNET_EMAILGEN_LLM      -> DECNET_REALISM_LLM
  DECNET_EMAILGEN_MODEL    -> DECNET_REALISM_MODEL
  DECNET_EMAILGEN_TIMEOUT  -> DECNET_REALISM_TIMEOUT
  DECNET_EMAILGEN_PERSONAS -> DECNET_REALISM_PERSONAS
  DECNET_EMAILGEN_FAKE_OUTPUT -> DECNET_REALISM_FAKE_OUTPUT

Importers rewritten in: orchestrator/emailgen/scheduler.py,
orchestrator/drivers/email.py, web/router/{emailgen,topology}/
api_personas.py, cli/emailgen.py. Tests for moved modules relocated
to tests/realism/; tests for stay-put modules updated in place.

API URL `/api/v1/emailgen/personas` and CLI `decnet emailgen
import-personas` keep their public names until the service-collapse
commit (stage 5).

decnet/web/router/topology/__init__.py

@@ -21,6 +21,7 @@ from .api_get_topology import router as _get_router
 from .api_lan_crud import router as _lan_router
 from .api_list_topologies import router as _list_router
 from .api_mutations import router as _mutations_router
+from .api_personas import router as _personas_router
 from .api_reap_orphans import router as _reap_router
 from .api_teardown_topology import router as _teardown_router
 
@@ -44,6 +45,10 @@ topology_router.include_router(_decky_router)
 topology_router.include_router(_edge_router)
 topology_router.include_router(_mutations_router)
 topology_router.include_router(_events_router)
+# Personas use a literal-suffix path (`/{id}/personas`) — register
+# before the bare `/{id}` getter so FastAPI's trie sees the literal
+# segment first.
+topology_router.include_router(_personas_router)
 topology_router.include_router(_get_router)