Tightens lint policy, documents API, and hardens runtime paths

[leynos]

Why

Adopting a more opinionated lint configuration surfaces latent defects,
encourages defensive coding, and provides users with better‐documented
APIs. The patch adds the missing documentation and refactors panic-prone
code so the project builds cleanly with the newly denied lints.

What

• Lint gate
  • Denies many additional clippy::* groups, missing_docs, and
    missing_crate_level_docs.
  • Lowers cognitive–complexity threshold to 9; allows expect only
    inside tests.
• Codebase adjustments
  • Adds or moves module roots (ir/, manifest/) to attach crate-level
    docs.
  • Replaces unchecked unwrap/expect in production code with
    fallible paths and rich error messages; keeps them in tests behind an
    explicit allowance.
  • Introduces targeted debug assertions and graceful fall-back logic
    (e.g. string conversions, thread joins, HTTP fixtures).
  • Switches eprintln! to tracing for structured logging.
  • Improves CLI path resolution and build‐script robustness.
• Documentation
  • Provides doc comments for public structs, enums, and fields that were
    previously undocumented.

Outcome

The crate now builds with cargo clippy --all-targets -- -D warnings,
has a clearer public surface, and is less prone to runtime panics or
invalid UTF-8 issues while preserving existing behaviour in tests and
features.