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>

tests/golden/help-cache.golden

@@ -0,0 +1,20 @@
+exit: 0
+--- stdout
+Manage local cache
+
+Usage: idx cache [OPTIONS] <COMMAND>
+
+Commands:
+  info   Show cache usage information
+  clear  Clear all cached files
+  help   Print this message or the help of the given subcommand(s)
+
+Options:
+  -o, --output <OUTPUT>  Output format [possible values: table, json]
+      --no-color         Disable colored output
+  -q, --quiet            Suppress non-essential output
+  -v, --verbose...       Increase verbosity (-v, -vv)
+      --offline          Run without network requests
+      --no-cache         Bypass local cache
+  -h, --help             Print help
+--- stderr