refactor: adopt MaybeSend portability vocabulary

[gold-silver-copper]

Adopt MaybeSend portability vocabulary

Description

This refactors Rig's cross-platform async portability vocabulary to conventional Rust naming while preserving the exact existing behavior.

• Rename WasmCompatSend, WasmCompatSync, and WasmBoxedFuture to MaybeSend, MaybeSync, and BoxFuture.
• Relax thread-safety only for all(target_arch = "wasm32", target_os = "unknown"); WASI and every other target retain real Send/Sync guarantees.
• Implement BoxFuture with futures::future::BoxFuture or LocalBoxFuture.
• Remove the HTTP-only stream marker in favor of target-specific BoxStream/LocalBoxStream aliases.
• Audit portable provider, tool, hook, memory, vector-store, erased-object, boxed-error, future, and stream bounds.
• Add core and root-facade compile contracts plus native regression coverage.
• Update public documentation, contributor guidance, migration notes, and the changelog.

This is intentionally a breaking API rename with no compatibility shims.

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation update

Testing

• cargo fmt --all --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test
• cargo check -p rig-core --target wasm32-unknown-unknown
• cargo check -p rig-agent --target wasm32-unknown-unknown
• cargo check -p rig --target wasm32-unknown-unknown
• cargo doc --workspace --no-deps
• Targeted core/facade/rig-memory checks and stale-identifier audits

Checklist

• My code follows the style guidelines of this project
• I have commented code where the target-specific behavior needs explanation
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove the portability contract
• New and existing unit tests pass locally with my changes

Notes

A fresh independent review covered the complete diff. Its one finding—a stale compatibility name in a comment—was fixed, affected checks were rerun, and the final independent pass found no remaining actionable issues.

Two broader pre-existing target limitations remain outside this PR's scope:

• rig-core --all-features on browser WASM is blocked by lopdf → getrandom without wasm_js.
• WASI compilation is blocked earlier by existing Tokio feature and socket2 target support.