feat: add unbiased integer range helpers

Expand range sampling from the original u32-only helper to all primitive
integer widths. The new gen_range_<int> methods cover 0..n sampling, while
matching gen_range_<int>_in methods accept standard RangeBounds forms such as
exclusive, inclusive, open-ended, and full-width ranges.

Use rejection sampling against the matching primitive reader for each integer
width. That keeps modulo bias out of the public helpers without introducing a
general RNG trait or depending on rand. Full-width ranges fall back directly to
the primitive reader because their span cannot be represented in the same
integer type.

Document the generated APIs with doctested examples and add a ranges example
that demonstrates unsigned, signed, usize, and full-width sampling.

Test Plan:
- cargo test
- cargo clippy
- cargo clippy --benches
- cargo clippy --tests
- cargo +nightly fmt

Refs: IDEAS.md ergonomics backlog

examples/demo.rs

@@ -11,6 +11,8 @@ fn main() -> std::io::Result<()> {
     println!("i32      : {}", rng.get_i32()?);
 
     println!("dice 1-6 : {}", rng.gen_range_u32(6)? + 1);
+    println!("offset   : {}", rng.gen_range_i32_in(-10..=10)?);
+    println!("index    : {}", rng.gen_range_usize_in(0..16)?);
 
     let token = rng.string_from(charset::ALPHANUMERIC, 24)?;
     println!("token    : {token}");