chore: add CI, crate metadata, and crate-level documentation

There was no CI of any kind — no workflows directory at all — despite a full
release pipeline (release.toml, cliff.toml, a maintained CHANGELOG, three
tagged releases). The workflow covers the feature combinations that actually
have distinct behaviour, including a release-profile job: `debug_assert!` is
compiled out there, which is exactly where the validation this branch added
has to hold, and a debug-only suite would never have seen it. Determinism is
checked at RAYON_NUM_THREADS of 1, 2, 4 and 8.

The Justfile gains test/lint/fmt/determinism recipes so the same checks run
locally with one command, and `just ci` runs the lot.

`Cargo.toml` had only name, version and edition, so `cargo publish` would
have been rejected. Added description, repository, readme, keywords,
categories, exclude, and `rust-version = "1.85"` — the edition-2024 floor,
now verified by a CI job. Two let-chains introduced earlier on this branch
would have pushed that to 1.88; they are rewritten to keep the floor where
it was.

`src/lib.rs` had no `//!` header at all, so the docs.rs landing page would
have been a bare symbol list — conspicuous given every other module has one.
It now explains what Through Time does differently, and carries three
runnable examples (which `cargo test --doc` checks, where previously there
was nothing to check), including the draw/p_draw interaction that is the
easiest way to get an error out of this crate.

`cargo publish --dry-run` now packages and verifies cleanly. The only
remaining blocker is `license`, which is yours to choose — noted as a TODO
in the manifest rather than picked unilaterally.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DnsaJg74eNSva3PJjK2eej

Justfile

@@ -1,4 +1,39 @@
 alias b := bench
+alias t := test
+
+# Run the full test suite across the feature combinations CI checks.
+test:
+    cargo test
+    cargo test --features approx
+    cargo test --features approx,rayon
+    cargo test --release --features approx
+
+# Fast inner-loop tests.
+check:
+    cargo test --features approx
+
+# Posteriors must be bit-identical across rayon worker counts.
+determinism:
+    #!/usr/bin/env bash
+    set -euo pipefail
+    for threads in 1 2 4 8; do
+        echo "== RAYON_NUM_THREADS=$threads =="
+        RAYON_NUM_THREADS=$threads cargo test --release \
+            --features approx,rayon --test determinism
+    done
+
+lint:
+    cargo clippy --all-targets --all-features -- -D warnings
+
+# Always nightly: rustfmt.toml uses nightly-only options.
+fmt:
+    cargo +nightly fmt
+
+fmt-check:
+    cargo +nightly fmt --check
+
+# Everything CI runs.
+ci: fmt-check lint test determinism
 
 store:
     cargo bench -- --save-baseline base