refactor: split into idx-core library and idx-cli binary workspace

Phase 2 of docs/DESIGN.md, no behavior change:

- crates/idx-core (library): api, analysis, cache, config file handling
  and provider selection, curl_impersonate, error, ownership, runtime,
  plus the shared test fixtures. clap derives are behind an optional
  `clap` feature so the library does not require clap.
- crates/idx-cli (binary `idx`): cli/, output/, and IdxConfig (output
  format and color resolution). Package and binary names are unchanged.
- config.rs is split: file read/write and validation move to core; a
  CLI test keeps core's OUTPUT_VALUES in sync with OutputSetting.
- ownership::db::open_db/db_path drop their unused IdxConfig argument.
- One workspace version; shared dependency versions; no third-party
  version changes in Cargo.lock. LICENSE is symlinked into each crate.
- flake: workspace version, new source paths, and a per-crate package
  check (idx-cli is built from its tarball against packaged idx-core,
  since idx-core is not on crates.io yet). CI cache key, release path
  filter, and the golden whitespace-hook exclusion follow the new layout.

Parity: all 143 golden outputs match byte for byte without
regeneration; 157 core + 17 CLI unit tests (173 before plus the new
OUTPUT_VALUES sync test), 62 CLI tests pass. Live A/B against main:
16/16 stocks cases identical; ownership discover got IDX 403 on both
(exit 4 vs 1 is the phase 1 exit-code change).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/golden/ownership-concentration.json.golden

@@ -1,4 +0,0 @@
-exit: 0
---- stdout
-[["AALI",{"top1_bps":7968,"top3_bps":7968,"hhi":6348,"free_float_bps":2032,"holder_count":1}],["BBCA",{"top1_bps":5494,"top3_bps":5716,"hhi":3021,"free_float_bps":4182,"holder_count":4}],["AADI",{"top1_bps":4110,"top3_bps":6230,"hhi":2009,"free_float_bps":2030,"holder_count":10}],["ABBA",{"top1_bps":283,"top3_bps":418,"hhi":9,"free_float_bps":9582,"holder_count":2}]]
---- stderr