feat: extract pure fiscal core from sowai-auto (parity-preserving copy)

Copies the 6 pure fiscal modules (domains, chave_acesso, tpag, presets,
resolver, xml_builder) from sowai-auto's backend/app/modules/fiscal/ into
this standalone lib, with only the two mechanical changes required to drop
the app.* dependency: import prefix rename, and TaxRule (ORM) -> TaxRuleLike
(structural Protocol) in the resolver. No other line changes -- byte-level
parity with the auto is the point.

Ports the pure test suites (domains, chave_acesso, resolver with a local
FakeTaxRule satisfying TaxRuleLike, xml_builder) plus a new test_presets_data
covering the PRESETS dict directly (the auto's equivalent test exercises the
apply-preset HTTP/DB flow, which isn't part of the extracted pure core).

55 tests pass locally via `uv run pytest`, no Postgres/k8s required.

.gitignore

@@ -0,0 +1,6 @@
+.venv/
+__pycache__/
+*.pyc
+.pytest_cache/
+dist/
+*.egg-info/