feat: extraction framework overhaul — extract_core shared package + SQLite state tracking

- Add extract/extract_core/ workspace package with three modules:
  - state.py: SQLite run tracking (open_state_db, start_run, end_run, get_last_cursor)
  - http.py: niquests session factory + etag normalization helpers
  - files.py: landing_path, content_hash, write_bytes_atomic (atomic gzip writes)
- State lives at {LANDING_DIR}/.state.sqlite — no extra env var needed
- SQLite chosen over DuckDB: state tracking is OLTP (row inserts/updates), not analytical
- Refactor all 4 extractors (psdonline, cftc_cot, coffee_prices, ice_stocks):
  - Replace inline boilerplate with extract_core helpers
  - Add start_run/end_run tracking to every extraction entry point
  - extract_cot_year returns int (bytes_written) instead of bool
- Update tests: assert result == 0 (not `is False`) for the return type change

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

extract/extract_core/pyproject.toml

@@ -0,0 +1,15 @@
+[project]
+name = "extract_core"
+version = "0.1.0"
+description = "Shared extraction utilities: SQLite state tracking, HTTP helpers, file I/O"
+requires-python = ">=3.13"
+dependencies = [
+    "niquests>=3.14.1",
+]
+
+[build-system]
+requires = ["hatchling"]
+build-backend = "hatchling.build"
+
+[tool.hatch.build.targets.wheel]
+packages = ["src/extract_core"]