refactor!: remove madsim support and runtime adapter

[MrCroxx]

Foyer now depends directly on upstream Tokio. Remove the simulation runtime dependency, the foyer-tokio adapter crate, runtime-selection features, conditional compilation, CI job, xtask command, and related changelog references.

Breaking change: remove runtime-tokio and runtime-madsim-tokio. Consumers should remove these feature selections; Tokio is used automatically, including with default-features = false.

Validation:

• Default and no-default-features workspace nextest runs with strict_assertions: 113 passed and 1 skipped each.
• Workspace doctests: 4 passed.
• Clippy across all workspace targets with strict assertions, serde, tracing, clap, and deadlock; tokio-console checked separately.
• Rust 1.91.0 minimum-version check for foyer without default features; isolated checks for common, memory, and storage crates.
• Stable/nightly formatting, TOML formatting, typos, cargo-machete, and license headers.
• Repository and resolved dependency graph checked for remaining simulation/runtime-adapter references.

The benchmark's existing tracing and tokio-console setup functions cannot be enabled together, so those configurations were checked separately as in CI.

Downstream simulation compatibility: RisingWave currently locks foyer 0.22.3 and enables runtime-madsim-tokio. Its existing locked build is unaffected, but migrating to this change requires a downstream simulation adapter. Removing the feature selection alone leaves foyer using real Tokio: a focused test reproduced Spawner::current() panicking inside a madsim runtime. A crates.io patch alias cannot replace the tokio package with madsim-tokio; a same-name forwarding shim also creates a dependency cycle because madsim itself depends on Tokio. The pinned madsim runtime additionally lacks shutdown_background(), which foyer now calls unconditionally.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
foyer-common/src/spawn.rs	74.00% <ø> (ø)
foyer-storage/src/io/engine/mod.rs	95.71% <ø> (ø)

... and 3 files with indirect coverage changes

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