Set up cargo workspace backbone

Two-crate workspace: immo-core for pure financial logic (wasm-compatible,
no async/IO deps) and immo-web for HTTP and rendering. Both crates are
still cargo-new skeletons; no domain code yet.

Pins the toolchain to 1.97 with rustfmt, clippy and the
wasm32-unknown-unknown target so the core crate's wasm constraint is
checkable locally. Cargo.lock is committed since the workspace ships a
binary.

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

.gitignore

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+# Build artifacts
+/target
+**/*.rs.bk
+*.pdb
+
+# Cargo.lock is committed: this workspace produces a binary (immo-web).
+
+# Coverage / profiling
+*.profraw
+*.profdata
+/tarpaulin-report.*
+/cobertura.xml
+
+# Editor and OS
+.DS_Store
+.idea/
+.vscode/
+*.swp
+
+# Local environment
+.env
+.env.local

CLAUDE.md

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+# CLAUDE.md
+
+## Project
+
+Web app that answers one question with data: for a given property in France, is buying
+financially better than renting and investing the same cash? It outputs an amortisation
+schedule, total ownership costs, and breakeven curves under several market hypotheses.
+
+This is a financial tool. A wrong number is worse than a slow response, an ugly page, or a
+missing feature. Correctness and traceable sources come first.
+
+I am learning Rust through this project. See "Working with me" below.
+
+## Commands
+
+```bash
+cargo run -p immo-web                   # start the server on :3000
+cargo watch -x 'run -p immo-web'        # live reload during development
+cargo test --workspace                  # all tests
+cargo test -p immo-core                 # domain tests only, fast
+cargo fmt --all
+cargo clippy --workspace --all-targets -- -D warnings
+```
+
+Run `cargo clippy` and `cargo test -p immo-core` before telling me a change is finished.
+
+## Architecture boundary
+
+Two crates. The boundary is a rule, not a description.
+
+`crates/immo-core` holds all financial logic as pure functions over plain structs.
+
+- No `tokio`, no `axum`, no `reqwest`, no filesystem, no network access.
+- Must stay compilable to `wasm32-unknown-unknown`. After adding any dependency, verify with
+  `cargo check -p immo-core --target wasm32-unknown-unknown`.
+- Time is a parameter. Never call `Utc::now()` or read the system clock inside `immo-core`.
+
+`crates/immo-web` holds HTTP, HTML rendering, and input parsing. It contains no financial
+arithmetic.
+
+If a computation is tempting to write inline in a handler, it belongs in `immo-core`.
+
+## Stack, pinned
+
+- axum 0.8. Path parameters use `{param}`, **not** `:param` — that is 0.7 syntax and will not compile.
+- `#[async_trait]` is not needed on `FromRequest` / `FromRequestParts` in 0.8. Do not add it.
+- askama for templates, tower-http for static files and compression, serde, rust_decimal.
+- Server-rendered HTML. No npm, no bundler, no JS framework. Charts are a CDN `<script>` tag fed
+  by a JSON endpoint.
+
+Before writing axum code from memory, check the version in `Cargo.toml` and verify the API against
+docs.rs for that exact version.
+
+## Domain rules
+
+- Money is `rust_decimal::Decimal` or integer cents. Never `f64`. Never `as` casts between numeric
+  types in financial code.
+- Rates are stored as annual nominal and converted explicitly. Name variables so the period is
+  unambiguous (`taeg_annual`, `monthly_rate`), never a bare `rate`.
+- Keep French domain vocabulary in type and field names: `FraisDeNotaire`, `TaxeFonciere`,
+  `IndiceReferenceLoyers`, `LoyerDeReferenceMajore`, `ZoneTendue`, `Taeg`. Do not translate them.
+- Every hardcoded assumption lives in `crates/immo-core/src/assumptions.rs` with a `///` comment
+  giving its source and the date it was checked. Nothing hardcoded anywhere else.
+- If you do not know a real figure — a barème, a tax rate, a fee schedule, an IRL value — say so and
+  leave a clearly named placeholder. Do not invent plausible-looking French tax numbers.
+- TAEG and taux nominal are different things. Distinguish them explicitly wherever both appear.
+
+## Error handling and tests
+
+- `thiserror` for typed errors in `immo-core`; `anyhow` only at the `immo-web` boundary.
+- No `unwrap()`, `expect()`, or `panic!()` outside tests and `main`.
+- Every `core` function producing a number the user sees has a known-answer test, with the worked
+  example stated in the test name or a comment above it.
+- Test the amortisation schedule against hand-checked values, never against the implementation's
+  own output.
+
+## Working with me
+
+I know traits, `Result`, and the borrow checker. I do not know the ecosystem well, and learning it
+is half the point of this project.
+
+- When you use a non-obvious idiom — extractors, `Layer`, lifetimes on a struct, `impl Trait` in
+  return position — explain why in one or two sentences, in chat rather than as a code comment.
+- If my request implies an unidiomatic design, say so before implementing it.
+- Prefer the simple version over the clever one, and tell me when you deliberately chose the
+  simple one.
+
+## Do not
+
+- Add a dependency without asking first, including small ones.
+- Introduce npm, a bundler, TypeScript, or a frontend framework.
+- Write financial logic in `crates/immo-web`.
+- Leave commented-out code, or comments that restate what the code does.
+- Reformat or restructure files you were not asked to touch.

Cargo.lock

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+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 4
+
+[[package]]
+name = "immo-core"
+version = "0.1.0"
+
+[[package]]
+name = "immo-web"
+version = "0.1.0"

Cargo.toml

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+[workspace]
+members = ["crates/immo-core", "crates/immo-web"]
+resolver = "3"
+
+[workspace.package]
+edition = "2024"
+rust-version = "1.97"

crates/immo-core/Cargo.toml

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+[package]
+name = "immo-core"
+version = "0.1.0"
+edition.workspace = true
+rust-version.workspace = true
+
+[dependencies]

crates/immo-core/src/lib.rs

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+pub fn add(left: u64, right: u64) -> u64 {
+    left + right
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn it_works() {
+        let result = add(2, 2);
+        assert_eq!(result, 4);
+    }
+}

crates/immo-web/Cargo.toml

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+[package]
+name = "immo-web"
+version = "0.1.0"
+edition.workspace = true
+rust-version.workspace = true
+
+[dependencies]

crates/immo-web/src/main.rs

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+fn main() {
+    println!("Hello, world!");
+}

rust-toolchain.toml

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+[toolchain]
+channel = "1.97"
+components = ["rustfmt", "clippy"]
+targets = ["wasm32-unknown-unknown"]