fix: gate sleepable probes behind opt-in

[swananan]

Summary

• skip sleepable uprobe helper and tail-call probes unless the mode is explicitly enabled
• preserve the published one-argument startup API and add an explicitly named sleepable opt-in entry point
• keep separate base and full capability caches so default startup cannot hide later opt-in detection
• add end-to-end coverage that evicts an anonymous page with MADV_DONTNEED, proves an ordinary uprobe cannot read it, and verifies sleepable mode faults it back in
• preserve existing source fixture line numbers used by line-level tracing tests

Testing

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• CARGO_TARGET_DIR=/tmp/ghostscope-target-gate-sleepable cargo test -p ghostscope-loader kernel_caps
• full standard host e2e suite through the runner service (job 4a4f9d0c2d74)