feat(arty_io_core): add injectable I/O driver contracts

[martintmk]

Summary

Introduces the initial stable arty_io_core contract for injecting I/O drivers after runtime startup.

• DriverContext selects its associated provider, so get_context::<MyContext>() needs no registration value.
• The first lookup initializes one driver instance on every active worker before returning the caller-local context.
• Later lookups reuse the cached context without creating more drivers.
• Multiple unrelated and semver-incompatible driver types can coexist in one runtime.
• Driver remains thread-local, boxable as Box<dyn Driver<Context = C>>, and exposes only shared-reference callbacks.
• Parker defines the latched wait/wake contract.
• SystemTasks is a cloneable crate-owned handle that hides runtime shared-ownership plumbing.
• Shutdown is initiated exactly once by the runtime and returns an object-safe boxed completion future.
• Contexts and in-flight operations retain state through safe ownership handles rather than an unsafe is_inert contract.

The crate includes REQUIREMENTS.md, DESIGN.md, public-surface tests, and a modular two-worker example. The example registers two independent driver types after worker startup, performs in-memory I/O through both contexts, caches repeated lookups, drains all four driver instances, and rejects operations through retained contexts after shutdown.

Review

• Multi-model reviews with Opus, Sonnet, and GPT found no remaining issues.
• Pragmatic Rust public API review found no remaining issues after removing speculative Clone from DriverInit and replacing exposed Arc<dyn ...> plumbing with SystemTasks.
• Package coverage is 100%.

Validation

• just package=arty_io_core clippy
• just package=arty_io_core format
• just package=arty_io_core readme
• just package=arty_io_core spellcheck
• cargo test -p arty_io_core
• cargo run -p arty_io_core --example two_thread_runtime
• cargo coverage-gate --package arty_io_core
• cargo public-api --all-features --manifest-path crates/arty_io_core/Cargo.toml

CI infrastructure blocker

The Windows coverage job fails after all 6,867 tests pass. cargo llvm-cov generates a 33,956-character llvm-cov.exe export command for 445 objects, exceeding the Windows 32,767-character process limit (os error 206). The failed job was retried once and reproduced identically. Linux and Linux ARM coverage, Codecov, mutation tests, static analysis, MSRV, semver, external-type exposure, spelling, and other completed checks pass.

[martintmk]

[AI AGENT]: I reviewed all eight added public contracts (Driver, DriverContext, DriverInit, DriverProvider, Parker, Shutdown, SystemTask, and SystemTaskSpawner), their rustdoc, the example runtime, dependencies, naming, object safety, shutdown behavior, and the new public-surface tests. Two targeted probes reproduced lifecycle contradictions below. The remaining public surface is documented and intentionally narrow; I found no additional dependency, naming, performance, resilience, or test-weakening issue. CI was still running with no reported failures at the final check.

[codecov]

Codecov Report

❌ Patch coverage is 94.23077% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.9%. Comparing base (efd8178) to head (02fd34a).

Files with missing lines	Patch %	Lines
crates/arty_io_core/src/system_tasks.rs	66.6%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #737     +/-   ##
========================================
- Coverage   100.0%   99.9%   -0.1%     
========================================
  Files         583     587      +4     
  Lines       62930   62982     +52     
========================================
+ Hits        62930   62979     +49     
- Misses          0       3      +3     

Flag	Coverage Δ
linux	99.9% <94.2%> (-0.1%)	⬇️
linux-arm	99.9% <94.2%> (-0.1%)	⬇️
scheduled	?

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[martintmk]

[AI AGENT]: Pragmatic Rust public API review completed with no findings. Verified the all-features cargo public-api surface, generated rustdoc JSON, package tests, clippy, spelling, the two-driver example, and the 100% coverage gate.

Coverage: reviewed Driver, DriverContext, DriverProvider, DriverInit, Parker, SystemTasks, and SystemTask from the downstream consumer and evolution perspective. The audit cannot prove runtime behavior or soundness from API output alone; those were covered separately by tests and multi-model review.

The resulting surface removes speculative Clone from DriverInit, hides Arc<dyn ...> behind the cloneable SystemTasks handle, and documents the boxed shutdown future as the deliberate object-safe shape for Box<dyn Driver<Context = C>>.