[kvlonge]

sl.http fan-outs can now nest: a fan_out can be another fan_out's over, as many levels deep as needed (orders → items → notes).

• Each key carries the fields of every level above it, so a level-3 path such as /orders/{order_id}/items/{item_id}/notes is filled with the right pair. A new optional key= on paginate and fan_out renames the key a level feeds down, for APIs that call every level's id id.
• Every level keeps its own paginator, concurrency, on_status, and (through the sl.http it comes from) its own retries and rate limit.
• Errors in nested jobs name the keys that led to them, such as /tree/orders order_id=5 → /tree/orders/5/items item_id=51 → GET …/notes?page=2: HTTP 500. Each breadcrumb step is redacted with the secrets of the level that made it. The retryable flag (exit 75) is preserved.
• Plan checks cover every level: every placeholder must be given by a level above, and a key name a level above already gives is an error (rename it with key=).
• An incremental cursor goes on the top level; lower levels only see the keys it picks, so a rerun requests nothing below it.
• Only the last level's rows load. A level that should be its own table is its own table function, which repeats the levels above it.

Existing jobs are unchanged. The job JSON for one-level fan-outs is pinned byte for byte. One-level error texts get no breadcrumb, and plan errors keep their wording and order (two ordered cases fail on the first version of this branch).

Docs: "Multi-level fan-out" in sources/python.mdx, marked as coming in 0.37.0.

Tests:

• packages/sqlload/tests/test_http_jobs.py, against a three-level stub API whose notes endpoint returns 404 for a wrong order/item pair:
  • exact rows and the exact set of requests at concurrency 3 per level;
  • a 404 at level 2, with and without on_status;
  • a 500 at level 3, with the full breadcrumb, exit 75, and per-level retries;
  • a cursor rerun that requests only the top level;
  • key= renames at two levels;
  • byte-identical one-level job JSON.
• crates/sqlload/src/http_source/tests/test_job_problems.rs: multi-level plan checks and the one-level ordering.

71 http job tests pass and make check is clean. Mutation check: dropping the ancestor fields from forwarded keys fails 4 tests.

Follow-ups, not in this PR: inner-level plan messages don't say which level they come from, and listed object keys count every common field as given.