docs(examples): add runnable demo example

Adds examples/demo.rs so users can quickly see the crate in action via
`cargo run --example demo` without writing any glue code first.

The example exercises the three main surface areas of the crate:

- the typed primitive readers (get_u8, get_u32, get_u64, get_i32) to
  show that every integer width is covered and that signed values come
  out signed;
- gen_range_u32 in a dice-roll idiom (+ 1 to shift 0..6 into 1..6),
  which doubles as a hint that the helper returns a half-open range;
- string_from with three of the prebuilt charsets (ALPHANUMERIC for a
  generic token, HEX_LOWER for a 128-bit-style hex string, DIGITS for
  a numeric PIN), demonstrating the typical "generate me a random
  identifier" use case the crate is designed for.

No library code changes; this is purely an onboarding aid. Picked an
example over expanding the crate-level rustdoc because a runnable
binary is easier to copy-paste-modify than a doctest, and Cargo's
examples/ convention is the idiomatic place for this.

Test Plan:
- `cargo run --example demo` prints one line per demonstrated API and
  exits 0.
- `cargo clippy --all-targets` is clean.

examples/demo.rs

@@ -0,0 +1,25 @@
+//! Run with: `cargo run --example demo`
+
+use ez_urandom::{OsRandom, charset};
+
+fn main() -> std::io::Result<()> {
+    let mut rng = OsRandom::try_new()?;
+
+    println!("u8       : {}", rng.get_u8()?);
+    println!("u32      : {}", rng.get_u32()?);
+    println!("u64      : {}", rng.get_u64()?);
+    println!("i32      : {}", rng.get_i32()?);
+
+    println!("dice 1-6 : {}", rng.gen_range_u32(6)? + 1);
+
+    let token = rng.string_from(charset::ALPHANUMERIC, 24)?;
+    println!("token    : {token}");
+
+    let hex = rng.string_from(charset::HEX_LOWER, 32)?;
+    println!("hex      : {hex}");
+
+    let pin = rng.string_from(charset::DIGITS, 6)?;
+    println!("pin      : {pin}");
+
+    Ok(())
+}