Add generated loan offer fixtures for known-answer tests

Four reference offers with their full amortisation schedules, 960 instalment
lines in total, emitted by scripts/loan_schedule.py --rust. The generator was
verified against a real 2020 Caisse d'Epargne amortisation table and
cross-checked against the ANIL simulator, so the expected values come from
outside this crate rather than from its own output.

Do not hand-edit loan_offer_fixture.rs; regenerate it.

The module is #[cfg(test)] so the fixtures never reach the wasm build. Nothing
consumes them yet.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

crates/immo-core/src/fixtures/mod.rs

@@ -0,0 +1 @@
+pub mod loan_offer_fixture;

crates/immo-core/src/lib.rs

@@ -1,2 +1,5 @@
 pub mod euros;
 pub mod loan_offer;
+
+#[cfg(test)]
+pub mod fixtures;