refactor(transform): remove raw layer, read landing zone directly

- Delete 6 data raw models (coffee_prices, cot_disaggregated, ice_*,
  psd_data) — pure read_csv passthroughs with no added value
- Move 3 PSD seed models raw/ → seeds/, rename schema raw.* → seeds.*
- Update staging.psdalldata__commodity: read_csv(@psd_glob()) directly,
  join seeds.psd_* instead of raw.psd_*
- Update 5 foundation models: inline read_csv() with src CTE, removing
  raw.* dependency (fct_coffee_prices, fct_cot_positioning, fct_ice_*)
- Remove fixture-based SQLMesh test that depended on raw.cot_disaggregated
  (unit tests incompatible with inline read_csv; integration run covers this)
- Update readme.md: 3-layer architecture (staging/foundation → serving)

Landing files are immutable and content-addressed — the landing directory
is the audit trail. A raw SQL layer duplicated file bytes into DuckDB
with no added value.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

transform/sqlmesh_materia/models/seeds/psd_commodity_codes.sql

@@ -0,0 +1,10 @@
+MODEL (
+  name seeds.psd_commodity_codes,
+  kind SEED (
+    path '$root/seeds/psd_commodity_codes.csv',
+    csv_settings (
+      delimiter = ';'
+)
+ )
+);
+