Make syscall policy explicit; rename deny to blocklist

[congwang-mk]

Summary

• Rust confine() API rename: drops the legacy spelling that mismatched the rest of the public API. (643097d)
• Make syscall policy explicit: replaces the implicit policy.allow_syscalls / policy.deny_syscalls pair with a single explicit SyscallPolicy { None, Allow(_), Deny(_) } enum so it's impossible for a policy to be in an underspecified middle state. (a5bc946)
• Remove unimplemented syscall allowlist: the Allow(_) arm was never wired into the BPF emitter — drop it rather than ship a knob that silently does nothing. (783350a)
• Rename syscall deny policy to blocklist: DEFAULT_DENY_SYSCALLS → DEFAULT_BLOCKLIST_SYSCALLS, SyscallPolicy::Deny → SyscallPolicy::Blocklist, deny_syscall_numbers → blocklist_syscall_numbers, builder method deny_syscalls → block_syscalls. Prose-level mentions of "deny JEQs" / "kernel-deny" stay since those describe the BPF concept, not the renamed list. (540aebe)

Rebased onto current main (the Handler-trait reshape from #36 plus its post-merge cleanup). Touches Rust core, FFI, CLI, Python, README, and docs/extension-handlers.md.

Test plan

• cargo build --workspace --tests clean
• cargo test --workspace --lib — 258 passing
• cargo test --workspace --test integration extra — 14 passing (full Handler-trait integration coverage)
• CI on Linux x86_64 + aarch64
• Python pytest on the new policy.py / _profile.py / _sdk.py shape