feat(data): add runtime currency data lookup behind data feature flag

[Copilot]

Exposes the compile-time ISO_CURRENCY_DATA PHF map at runtime via a new opt-in data feature, enabling users to query currency metadata dynamically by ISO 4217 code.

Changes

• Cargo.toml: Added data feature gating optional phf dependency; phf remains a build-dep for codegen
• data.rs → src/data.rs: Moved to standard src/ layout; added get(code: &str) -> Option<Data> function
• build.rs: Updated mod data with #[path = "src/data.rs"] to reflect new location
• src/lib.rs: Conditionally includes pub mod data under #[cfg(feature = "data")]
• src/data_test.rs: Tests for get() — known codes, unknown codes, case sensitivity, direct map access

Usage

// Cargo.toml: currencylib = { features = ["data"] }

use currencylib::data;

let usd = data::get("USD").unwrap();
println!("{} ({})", usd.name, usd.symbol); // "United States dollar ($)"

assert!(data::get("XYZ").is_none()); // unknown code returns None

[mfirhas]

@copilot Why don't we use include! and include data.rs from outside?, if you think your current implementation is better, then explain why and make no changes.

[Copilot]

@copilot Why don't we use include! and include data.rs from outside?, if you think your current implementation is better, then explain why and make no changes.

The current approach is preferable for a few reasons:

1. include! is idiomatic for generated code — in this codebase include! is used in iso_currencies.rs to pull in the build-generated file from OUT_DIR. Using it for a hand-written source file would be inconsistent with that established pattern.

2. data.rs is now a library source file — it was originally only a codegen input to build.rs, but after this change it's also compiled into the library itself. Source files that are part of the library belong in src/.

3. pub mod data; is cleaner — the standard Rust module declaration in lib.rs is simpler and more readable than pub mod data { include!("../data.rs"); } or a src/data.rs wrapper that just does include!("../data.rs").

4. #[path] in build.rs is the accepted Rust idiom for pointing a mod declaration at a non-default path. The one-line #[path = "src/data.rs"] mod data; in build.rs is a small, well-understood cost.

No changes made.

[mfirhas]

@copilot I want to confirm. The no_std is kept, unless user opt-in to data feature?

[Copilot]

@copilot I want to confirm. The no_std is kept, unless user opt-in to data feature?

Yes, confirmed. #![no_std] in lib.rs is unconditional — it applies whether or not data is enabled.

data.rs is also no_std-safe: phf is declared without its std feature, Data uses only &'static str fields, and get() returns Option<Data> — both from core. No std dependency is introduced by the data feature.

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