feat: v0.5.0 — comprehensive documentation release

Theme: documentation and project polish. No public-API changes; this
is the v0.5 release that elevates heuropt's docs/onboarding/governance
to bar-setting status.

Adds:
- mdbook user guide at docs/book/ with intro, getting-started,
  defining-problems, choosing-an-algorithm, cookbook (7 recipes),
  comparison vs other libraries, stability/SemVer, migration guides.
  Deploys to https://swaits.github.io/heuropt/ via .github/workflows/
  docs.yml.
- Runnable rustdoc examples on every algorithm (35 of them), all
  exercised by cargo test --doc.
- Three real-world examples: portfolio.rs (multi-obj with budget
  constraint), hyperparam_tuning.rs (BO + TPE), scheduling.rs
  (permutation via SA + SwapMutation against Smith's-rule oracle).
- Governance: CONTRIBUTING.md, SECURITY.md, CODE_OF_CONDUCT.md
  (adopting builderscode.org's Builder's Code of Conduct), GitHub
  issue templates, PR template.

Polishes:
- README hero with badges + user-guide link.
- lib.rs crate-level docs.
- CHANGELOG entry for 0.5.0.

Bumps Cargo.toml to 0.5.0.

.github/ISSUE_TEMPLATE/bug_report.md

@@ -0,0 +1,40 @@
+---
+name: Bug report
+about: A correctness, performance, or panic bug in heuropt
+title: "bug: <one-line summary>"
+labels: bug
+---
+
+## What happened
+
+<Concise description of the bug.>
+
+## Reproducer
+
+```rust
+// Smallest example that demonstrates the bug. Ideally <30 lines and
+// runnable as a fresh `examples/repro.rs`. Include the Cargo.toml
+// `[features]` you used.
+```
+
+Command used:
+
+```sh
+cargo run --release --example repro
+```
+
+## Expected vs observed
+
+- **Expected:** <what should happen>
+- **Observed:** <what actually happens>
+
+## Environment
+
+- heuropt version:
+- `rustc --version`:
+- OS / arch:
+- Feature flags enabled:
+
+## Additional context
+
+<Anything else — fuzz artifact path, screenshots, profiler output.>