feat(seccomp): ExtraHandler — user-supplied syscall handlers

[dzerik]

Summary

Adds a public extension point for downstream crates that need to register their own seccomp-notification handlers alongside sandlock's builtin chroot/cow/procfs/network/port_remap logic.

Motivation. Downstream crates that want to intercept additional syscalls in the same supervisor task as sandlock's builtins have no clean way to do it today — one SECCOMP_FILTER_FLAG_NEW_LISTENER per process means a single listener, so a second supervisor cannot run alongside. The only alternative is forking sandlock or patching notif::supervisor wholesale.

API.

• New type dispatch::ExtraHandler { syscall_nr, handler }.
• New entry Sandbox::run_with_extra_handlers(policy, cmd, extras).
• Existing Sandbox::run() delegates to it with empty extras — zero behaviour change for current callers.

Ordering contract (documented + tested).

• Builtins register first (chroot path normalization, COW, procfs, …).
• Extras appended last, in the Vec order.
• Chain stops at first non-Continue — user handlers cannot subvert builtin confinement.

Docs.

• docs/extension-handlers.md: design rationale, security boundary, panics policy, non-goals, downstream sketch.
• crates/sandlock-core/examples/openat_audit.rs: runnable example.

Minor bump 0.6 → 0.7 suggested.

Test plan

• 4 new unit tests in dispatch::extra_handler_tests (ctor, insertion order, append-after-builtin, empty-extras nop) — passing
• All 215 unit tests pass
• Example openat_audit.rs runs against a python3 -c guest