ci: tighten clippy/fmt/doc/audit + add GitHub Actions parity

Brings the workspace to clippy-clean under --all-targets -D warnings,
fixes broken benchmark targets, applies cargo fmt across the tree, and
mirrors the existing GitLab security/lint/test stages as GitHub Actions
so PR authors see the same checks before merge.

Clippy/lint fixes (across 15+ crates):
- Drop dead Node.level field in HNSW (level info lives in neighbors.len)
- Replace x % 2 != 0 with !x.is_multiple_of(2) (zkp.rs, auth.rs)
- Derive Default for PrivacyLevel, DriftPolicy, IsolationLevel
- Add Default impl for TransactionId, QualityReconciliationStrategy
- Rename TransactionId::from_str to from_string (avoids FromStr clash)
- Annotate too_many_arguments on octad/drift APIs (refactor would be
  invasive without semantic gain)
- Collapse nested ifs in vql.rs find_where_id, elide lifetimes
- Use struct-update syntax in OctadInput / RegenerationConfig builders
- Drop &h to h on Sha256::update (h is Copy)
- Fix HTTP URL literals in doc comments and unclosed HTML tags

Benchmark fixes:
- Switch from OxiGraphStore (now feature-gated, KNOWN-ISSUES #24) to
  SimpleGraphStore — drop-in pure-Rust replacement
- Add provenance + spatial stores to InMemoryOctadStore::new() calls
  (9 modalities now, was 7)
- Add verisim-provenance + verisim-spatial path deps to benches/
- Replace black_box(unit_returning_call) with explicit black_box(())

Doc warnings:
- Fix unresolved [`AdaptiveTuner`] link in profiler.rs
- Backtick the generic in `Version<T>` doc to avoid HTML parse
- Use backticks not quotes for example URLs

GitHub Actions added (.github/workflows/):
- rust-ci.yml: fmt, clippy (all-targets, -D warnings), test, doc
  (-D warnings via RUSTDOCFLAGS), cargo-audit, bench-compile
- elixir-ci.yml: format check, compile --warnings-as-errors, mix test,
  mix hex.audit, mix deps.unlock --check-unused

Dependabot fix:
- Correct ecosystem "weeklymix" → "mix" (was a typo, never matched)
- Point mix at /elixir-orchestration, npm at /playground (was scanning
  repo root, finding nothing)
- Drop pip + nix entries (no Python or flake files exist)

Verification:
- cargo test --workspace: 567 passed, 0 failed, 6 ignored
- cargo clippy --workspace --all-targets -- -D warnings: clean
- cargo doc --workspace --no-deps: clean
- cargo bench --no-run: clean

https://claude.ai/code/session_01GeiWCLLoZmPdnjMMcy2buu

Author: claude