MaxLLM v0.3.1 — regression-test suite, Rust 1.99 clippy fix, docker semver tag + manual CI trigger

What's in v0.3.1

Patch release on top of v0.3.0. The feature set is unchanged — this release hardens the
CI/release path and locks the current behaviour in with a large regression-test suite.

CI / release engineering

• ci(docker): the Docker image tag is now derived from the git tag instead of a hardcoded
  0.1.0, so a v0.3.1 tag yields the 0.3.1 image (plus latest and the commit-SHA tag).
• ci: added a workflow_dispatch trigger, so CI can be started manually on any ref
  (this fork's CI is dispatched manually on the release tag).
• fix(plugin): allow clippy::double_must_use on the #[async_trait] traits. Rust 1.99
  added that lint and it fires on the boxed futures generated by async_trait; CI clippy
  (-D warnings) is green again.

Test coverage (behaviour locked in)

• config: serde defaults and the validate() branches (unknown plugin / unknown provider
  references) — config mutation score 21.8% → 96.4%.
• plugin: auto-compaction end to end (truncate / sliding-window / LLM strategies, token
  estimation, apply_llm_compaction against a local HTTP mock).
• plugin: cache TTL / streaming / cacheable-status rules and the rate-limit key
  extraction (header, client IP, session).
• plugin: contract test over every built-in plugin category — name plus the configuration
  keys each category actually reads.
• plugin: guardrail engine (pass / block / log / modify, pre-call and post-call paths) and
  the built-in guardrail names.

Compatibility

No public API, configuration or wire-format changes: upgrading from v0.3.0 is drop-in.