refactor(firma-run): compile managed seccomp BPF via seccompiler

[lu-zero]

Why

crates/firma-run/src/seccomp.rs hand-emits classic-BPF opcodes byte-by-byte
(manual LD/JMP/RET encoding, manual arch-check prologue). That's the
riskiest part of the file to review and extend by hand. This is draft 1 of 2
evaluating a port to the seccompiler
crate (rust-vmm/Firecracker) as the BPF compiler backend; draft 2 will follow
up with a fork that adds public syscall-name-to-number resolution.

What Changed

• Replace emit_bpf_program/emit_stmt/emit_jump and the local
  TargetArch enum with seccompiler::SeccompFilter/SeccompAction/
  TargetArch, which generate the arch-check prologue and per-syscall jump
  table.
• Keep the existing hand-maintained SyscallId::number_for_arch per-arch
  syscall-number table as-is (released seccompiler has no public
  name-to-number resolution yet — that's what draft 2 addresses).
• Serialize the compiled Vec<sock_filter> to the same raw byte layout
  (u16 code, u8 jt, u8 jf, u32 k, native endian) the artifact file
  and bwrap --seccomp <fd> already expect, so the artifact/metadata/
  checksum contract in linux-local-command-enforcement.md is unchanged.
• Bump compiler_version to managed-seccomp-v2-seccompiler since the
  emitted BPF byte sequence differs from the hand-rolled emitter (same
  semantics, different instruction encoding from seccompiler's compiler).

Manual / Extra Verification

• Ran on an aarch64 Linux host: scripts/seccomp/check-managed-compatibility.sh passes.
• cargo nextest run -p firma-run (384 tests) passes, including the existing
  BPF-reproducibility test (compile_on_launch_is_reproducible_for_same_policy_and_arch)
  unmodified.

Risks / Notes

• No config, wire-format, or policy-input change. The set of denied syscalls
  per action class is identical; only how the BPF bytes for that deny-list
  get generated changed.
• Adds seccompiler (Apache-2.0 OR BSD-3-Clause, pure Rust, no system/link
  dependency) as a Linux-only dependency of firma-run.
• Confirmed dprint check is clean for every file this PR touches
  (Cargo.toml, crates/firma-run/Cargo.toml, crates/firma-run/src/seccomp.rs,
  docs/architecture/linux-local-command-enforcement.md). just fmt still
  fails repo-wide on 10 pre-existing files under todo/, unrelated to this
  change and already failing on main.

AI Assistance

Implemented with Claude Sonnet 5 (Claude Code) per explicit user request, after a
prior conversation turn reviewed the current seccomp implementation and
evaluated seccompiler vs libseccomp as replacement backends.