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/feature_request.md

@@ -0,0 +1,39 @@
+---
+name: Feature request
+about: Propose a new algorithm, operator, metric, or API addition
+title: "feat: <one-line summary>"
+labels: enhancement
+---
+
+## What and why
+
+<What you want, and the problem it solves. If this is a new algorithm
+or operator, cite the paper or canonical reference.>
+
+## Proposed API sketch
+
+```rust
+// What the public surface would look like — config struct fields,
+// trait impl, etc. Doesn't need to be final, just enough to discuss.
+```
+
+## Alternatives considered
+
+<Other approaches you thought about and why this one wins. If a
+similar feature already exists in heuropt or another Rust crate,
+explain how this differs.>
+
+## Scope
+
+- [ ] New trait (will need API discussion)
+- [ ] New algorithm
+- [ ] New operator
+- [ ] New metric / Pareto utility
+- [ ] New optional feature flag
+- [ ] Change to existing public API (potentially breaking)
+
+## Willing to implement?
+
+- [ ] Yes, I'll send a PR.
+- [ ] Yes, but I'd like guidance on the design first.
+- [ ] No, I'm reporting the need.