fix(sqlfluff): honor fallback rule, deterministic ordering, lint_cmd wiring

Address the second-round review on PR #253:

- JSON parse failure now passes raw sqlfluff output through unchanged,
  with an stderr warning, honoring the non-negotiable fallback rule
  (rust-patterns.md) instead of emitting a truncated hybrid summary.
- "first at" rule locations are derived from the original sqlfluff JSON
  order, not the sorted worst-first file list, so the sample points at the
  earliest occurrence rather than the first file in the report.
- All report sorts are now deterministic: tied rule counts break
  alphabetically by rule code, tied files by path.
- rtk lint sqlfluff now routes through the shared structured filter, with
  the same format-flag handling as sqlfluff_cmd::run; a user-supplied
  non-JSON format keeps raw (truncated) output.
- Document the joined short-flag gap (-fhuman) as a known limitation to
  be fixed with the arg-tokenizer migration (#3681).
- Add regression tests for fallback passthrough (incl. large, uncapped),
  original-order first-location, and deterministic tie ordering.

Pre-existing, out of scope, tracked separately: ruff_cmd's identical
fallback deviation, a shared compact_path/grouping helper across the six
duplicated modules, and the bare-sqlfluff discover rule gap (same as ruff).