test: add golden output parity gate and live A/B compare

Phase 0 of docs/DESIGN.md, before any refactor code:

- tests/golden.rs runs every case in tests/golden/cases.toml (126
  outputs across help, completions, config, cache, all stocks commands
  on MSN and Yahoo mocks, error paths, and ownership queries on a seeded
  fixture DB) and compares exit code, stdout, and stderr byte for byte.
  Regenerate only with UPDATE_GOLDEN=1 for a reviewed output change.
- scripts/live-smoke.sh --compare <old-idx> runs each live case with
  both binaries and fails on differing exit codes, added/missing/retyped
  JSON keys, or changed values for slow-moving data.
- Exclude golden files from the whitespace/EOF pre-commit fixers, which
  would otherwise rewrite pinned help output.

Both gates were fault-injected: a one-word table change fails exactly
the three history goldens, and a binary dropping profile.long_name fails
only the profile live comparison.

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

docs/DESIGN.md

@@ -162,7 +162,8 @@ is another 20-30% smaller than pretty JSON.
 - stdout carries only data; diagnostics and warnings go to stderr. Already true.
 - Errors in `json`/`jsonl` mode are one JSON object on stderr with stdout empty.
   Already true.
-- No ANSI color when stdout is not a terminal.
+- No ANSI color when stdout is not a terminal. Today some table headers emit
+  bold codes even when piped.
 - Commands that take symbols accept `-` to read them from stdin, one per line or
   whitespace-separated:
   `idx stocks screen --filter low-pe | jq -r '.[].symbol' | idx stocks compare -`
@@ -264,14 +265,16 @@ Before any refactor code lands, capture the current CLI as golden files:
   fixture-built ownership DB
 - the documented error cases (invalid input, unsupported provider, offline
   without cache, empty ownership DB)
-- stored as `tests/golden/<case>.{stdout,stderr,exit}` and compared byte for
-  byte by one data-driven test in `tests/golden.rs`
+- cases listed in `tests/golden/cases.toml`; each result stored as
+  `tests/golden/<case>.<format>.golden` (exit code, stdout, stderr) and
+  compared byte for byte by one data-driven test in `tests/golden.rs`
 
 Regenerating goldens (`UPDATE_GOLDEN=1`) is only allowed for an intentional,
 reviewed output change, and the diff must be in the same commit with the reason
-in the message. Phases 1-3 must not change any golden file. Phase 0 changes only
-what it documents: default format resolution, JSON whitespace when piped, and
-exit codes; JSON content and table bytes stay identical.
+in the message. Phases 2-4 must not change any golden file. Phase 1 changes only
+what it documents: default format resolution, JSON whitespace when piped, ANSI
+color when piped (table output emits bold headers into pipes today), and exit
+codes; JSON content and table text stay identical.
 
 `tests/cli.rs` stays the behavioral suite and must keep passing unchanged.