Add builder generation trace metadata

docs/prompt-pool-routing-map.md

@@ -221,6 +221,10 @@ Common trap: `row_number` participates in `seed_config.axis_rng`. If two
 workflows have the same seeds but different `row_number`, they are not expected
 to match.
 
+Each generated row stores `generation_trace.seed_axes` in `metadata_json`.
+Use it to verify whether an axis followed the main seed or a configured seed,
+and to compare the exact per-axis RNG seed used for the row.
+
 ## Category Sources
 
 There are two category systems.
@@ -513,6 +517,7 @@ plain prompt text. When debugging, inspect these fields before editing pools.
 | Field | Owner | Consumed by | Meaning |
 | --- | --- | --- | --- |
 | `source` | `build_prompt` / row builder | All formatters | Usually `json_category` or `built_in_generator`; tells which route created the row. |
+| `generation_trace` | `builder_prompt_route.build_prompt_result` | Debug | Compact generation route trace containing builder branch, input/resolved category, row seed, per-axis seed sources/RNG seeds, effective clothing/pose/figure choices, expression route, and content seed axis. |
 | `main_category`, `subcategory` | `row_category_route.select_category_item_route` | All formatters and debug | Human-readable selected category route. |
 | `category_slug`, `subcategory_slug` | `row_category_route.select_category_item_route` | Debug/filtering | Stable-ish machine labels for selected category route. |
 | `content_seed_axis` | `row_category_route.select_category_item_route` | Debug | Shows whether the item/action was driven by `content` or `pose`. Critical for hardcore pose categories. |