v1.0.4

feat: support organizing metrics into folders (#20) (#21)

sigma_data_models.folder() only accepted `columns` - there was no way to
group a table's metrics into folders, so metrics always rendered flat in
the Sigma UI regardless of how many a table had.

- folder.sql: folder() now takes an optional `metrics` param alongside
  `columns`.
- table.sql: tracks a `metric_ids` map (parallel to the existing
  `column_ids`) while composing frozen metrics, then resolves
  folder.metrics against it the same way folder.columns resolves against
  column_ids - items end up columns first, then metrics, each in declared
  order. An unknown metric reference raises a compiler error at compile
  time, mirroring the existing unknown-column folder error.
- Docs: macros/schema.yml and README.md updated for the new `metrics` arg.
- Tests: assert_sigma_spec.sql gets shape assertions for a folder mixing
  columns+metrics and a metrics-only folder, plus a new
  assert_unknown_folder_metric_rejected negative case (registered in
  scripts/run_integration_tests_duckdb.sh).

Verified via ./scripts/run_integration_tests_duckdb.sh - all existing
assertions (including the pinned frozen-id regression guard) plus the
new ones pass.

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>