IB-WP-0016-T06: OpenRouter live-run guardrails

Add --chapter / --from-chapter / --to-chapter / --chunk selection flags
to generate init and generate from-source, plumb them into
init_generation_infospace via a new _filter_chunks_by_chapter helper,
and refuse to create an infospace when the filters reject every chunk
(InfospaceError "empty_chapter_selection"). The flags use the same
T03/T02 plumbing (chapter labels, roman numerals, chunk ids) so a
single-chapter selection is a one-flag command.

OpenRouter run-record metadata (model, request_id, usage tokens,
retry_count, duration_seconds) already lands in
output/workflows/runs/*.yaml; this task just adds the smoke test that
proves it stays there, plus the parallel guarantee that the same
provider metadata reaches generated artifact provenance via
provenance.provider_metadata.

tests/test_openrouter_live.py covers:
- chapter-filter, from/to-chapter range, and empty-selection failure on
  init (non-live, deterministic)
- CLI smoke through generate from-source with --chapter
- a pytest-skipped live OpenRouter one-chapter end-to-end gated by
  OPENROUTER_API_KEY + INFOSPACE_BENCH_ENABLE_LIVE_OPENROUTER, with
  INFOSPACE_BENCH_LIVE_MODEL override (default openai/gpt-4o-mini)

docs/generic-source-generator.md gains a "Live OpenRouter runs (handle
with care)" section that walks plan-before-run, single-chapter live
run, the budget/usage artifacts, and the checks a reviewer should run
before scaling to the full book.

129 tests pass, 1 skipped (the live smoke, correctly gated).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

workplans/IB-WP-0016-lefevre-ebook-infospace-readiness.md

@@ -192,7 +192,7 @@ state_hub_task_id: "c9bbc84e-691b-4530-a79a-6ecfa9c41fdd"
 
 ```task
 id: IB-WP-0016-T06
-status: todo
+status: done
 priority: high
 state_hub_task_id: "c6bf97c3-1c2c-4993-8f4f-97a48e01cce2"
 ```