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/ranges.rs

@@ -0,0 +1,21 @@
+//! Run with: `cargo run --example ranges`
+
+use ez_urandom::OsRandom;
+
+fn main() -> std::io::Result<()> {
+    let mut rng = OsRandom::try_new()?;
+
+    let byte = rng.gen_range_u8(10)?;
+    let port = rng.gen_range_u16_in(49152..=65535)?;
+    let index = rng.gen_range_usize_in(0..8)?;
+    let offset = rng.gen_range_i32_in(-10..=10)?;
+    let full_width = rng.gen_range_i128_in(..)?;
+
+    println!("u8 below 10       : {byte}");
+    println!("ephemeral port    : {port}");
+    println!("usize index       : {index}");
+    println!("signed offset     : {offset}");
+    println!("full-width i128   : {full_width}");
+
+    Ok(())
+}