feat: add byte-fill convenience construction

Add OsRandom::fill_bytes so callers can fill a byte slice without importing
std::io::Read. This keeps the common raw-byte use case close to the rest of
the crate's typed convenience API while still forwarding to the same
/dev/urandom handle.

Implement Default for OsRandom as the infallible convenience constructor. The
fallible OsRandom::try_new API remains the right choice when the caller wants
to handle an unavailable operating-system randomness source explicitly.

Document the public constructors and byte-fill helper with working examples,
and add a runnable bytes example for the top-level workflow.

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

Refs: IDEAS.md ergonomics backlog

src/lib.rs

@@ -4,6 +4,16 @@
 //! `/dev/urandom` with helpers for reading primitive integers, sampling
 //! uniformly without modulo bias, and generating random ASCII strings from a
 //! caller-supplied alphabet. Common alphabets live in [`charset`].
+//!
+//! # Project layout
+//!
+//! - [`OsRandom`] owns an open `/dev/urandom` handle.
+//!   - Constructors open or duplicate that handle.
+//!   - Primitive readers turn raw bytes into integer values.
+//!   - Convenience byte readers fill caller-provided buffers.
+//! - Sampling helpers build unbiased higher-level choices from those primitive
+//!   readers.
+//! - [`charset`] contains reusable ASCII alphabets for string generation.
 
 use std::{
     fs::File,
@@ -102,17 +112,70 @@ macro_rules! os_random_get_integer_impls {
 impl OsRandom {
     /// # Errors
     /// Returns any I/O error produced while opening `/dev/urandom`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # fn main() -> std::io::Result<()> {
+    /// let mut rng = ez_urandom::OsRandom::try_new()?;
+    /// let value = rng.get_u64()?;
+    /// # let _ = value;
+    /// # Ok(())
+    /// # }
+    /// ```
     pub fn try_new() -> Result<Self, io::Error> {
         let devurandom = File::open(DEV_URANDOM)?;
 
         Ok(Self { devurandom })
     }
 
+    /// Fills `buf` with bytes from `/dev/urandom`.
+    ///
+    /// This is equivalent to calling [`Read::read_exact`] on `OsRandom`, but it
+    /// keeps the common "fill this byte slice" use case available without
+    /// importing the [`Read`] trait.
+    ///
+    /// # Errors
+    /// Returns any I/O error produced while reading from `/dev/urandom`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # fn main() -> std::io::Result<()> {
+    /// let mut rng = ez_urandom::OsRandom::try_new()?;
+    /// let mut bytes = [0u8; 32];
+    /// rng.fill_bytes(&mut bytes)?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn fill_bytes(&mut self, buf: &mut [u8]) -> io::Result<()> {
+        self.devurandom.read_exact(buf)
+    }
+
     os_random_get_integer_impls!(
         u8, u16, u32, u64, u128, usize, i8, i16, i32, i64, i128, isize
     );
 }
 
+/// Opens `/dev/urandom` and panics if the operating-system source is
+/// unavailable.
+///
+/// Prefer [`OsRandom::try_new`] when the caller should decide how to handle an
+/// I/O failure.
+///
+/// # Examples
+///
+/// ```
+/// let mut rng = ez_urandom::OsRandom::default();
+/// let mut bytes = [0u8; 16];
+/// rng.fill_bytes(&mut bytes).expect("/dev/urandom read failed");
+/// ```
+impl Default for OsRandom {
+    fn default() -> Self {
+        Self::try_new().expect("/dev/urandom should be available")
+    }
+}
+
 /// Uniform sampling helpers.
 ///
 /// All methods here use rejection sampling on top of [`OsRandom::get_u32`]