IB-WP-0019-T03: rate-table cost computation

Ship a starter model rate table at src/infospace_bench/model_rates.yaml
(prompt_per_1k / completion_per_1k for the OpenRouter models we have
actually touched: gpt-4o, gpt-4o-mini, gpt-4-turbo, claude 3.5 sonnet
and haiku, claude 3 opus, gemini 1.5 flash/pro, llama 3.1 70b) and a
load_rate_table() / estimate_cost_usd() pair that overlays an optional
<workspace>/model-rates.yaml on top of the bundled defaults.

generate run now passes a workspace-aware cost_resolver into
record_run_usage, so cost_usd_estimated lands on every usage bucket
whose model matches the table. Adapter-returned cost still wins
(cost_status="known"); rate-table cost is reported under
cost_status="estimated"; unmatched models are recorded as
cost_status="unknown" rather than silently zeroed. Rate-table file is
listed in pyproject.toml package-data so pip-installed users keep the
defaults.

106 tests pass.

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

src/infospace_bench/generator.py

@@ -20,6 +20,7 @@ from .lifecycle import create_infospace, load_infospace, register_artifact
 from .openrouter import OpenRouterAssistedGenerationAdapter
 from .budget import (
     latest_plan_snapshot_id,
+    make_cost_resolver,
     record_plan_snapshot,
     record_run_usage,
 )
@@ -324,6 +325,15 @@ def _read_profile_name(root: Path) -> str:
     return str(state.get("profile") or DEFAULT_PROFILE)
 
 
+def _workspace_for(root: Path) -> Path:
+    """Resolve the workspace directory that contains this infospace.
+
+    The standard layout is ``<workspace>/infospaces/<slug>``, so the
+    workspace is two levels above the infospace root.
+    """
+    return root.parent.parent
+
+
 def run_generation(
     root: str | Path,
     *,
@@ -396,6 +406,7 @@ def run_generation(
             snapshot_id=latest_plan_snapshot_id(root_path),
             duration_seconds=duration_seconds,
             started_at=started_wall.isoformat(),
+            cost_resolver=make_cost_resolver(_workspace_for(root_path)),
         )
     return GenerationRunResult(
         root=str(root_path),