ptx-schedule: classify grid dependency control sites

[uurl]

Closes #1207

Summary

Classify programmatic dependent launch control instructions as schedule-sensitive sites in ptx-schedule.

This adds a dedicated SiteKind::GridDependency for:

• griddepcontrol.launch_dependents
• griddepcontrol.wait

These instructions order execution and memory visibility between dependent grids, so schedule perturbation should be able to target them directly.

Why a separate site kind

griddepcontrol.* represents grid-to-grid ordering and is distinct from the asynchronous proxy pipeline currently modeled by SiteKind::AsyncProxy.

Keeping it separate also avoids folding grid-level dependency semantics into thread/CTA barriers or memory fences.

Tests

Added coverage that verifies:

• both backend-emitted griddepcontrol spellings are discovered;
• both are classified as SiteKind::GridDependency;
• grid dependency sites can be selected and perturbed with nanosleep.u32.

Validation:

cargo fmt --all --check
cargo test -p ptx-schedule grid_dependency
cargo test -p ptx-schedule
cargo clippy -p ptx-schedule --all-targets -- -D warnings
RUSTDOCFLAGS="-D warnings" cargo doc --no-deps -p ptx-schedule

All checks pass.