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.
