feat(gdb): debug guest kernel of a restored microVM

[kalyazin]

Why

Upstream wires gdb only into the boot path, so a snapshot-resumed microVM can't be
debugged — and the stub isn't all-stop, so multi-vCPU inspection hangs. We need to gdb
the guest kernel of a resumed snapshot (investigate resume behavior / slow
envd-init) with full source-level symbols, KASLR on.

What (all gdb code behind --features gdb; default/prod build untouched)

• start gdb on restore — accept a gdb_socket_path restore-time override on the
  load-snapshot request; build_microvm_from_snapshot wires attach_debug_info +
  gdb_thread at the restored RIP. Env fallback FIRECRACKER_GDB_SOCKET.
• all-stop — pause sibling vCPUs on every stop (QEMU-style) so info threads /
  per-vCPU backtraces work.
• drain stale debug events on resume — fixes a latent multi-vCPU race that dropped
  the gdb connection (GdbQueueError) under a breakpoint storm.
• snapshot-editor: print saved MSRs — so the KASLR slide can be recovered from a
  snapshot (MSR_LSTAR).
• integration test — restore (file/UFFD, 4K/2M hugetlb) on the prod DWARF kernel,
  recover the slide, hit a breakpoint, read kernel structs, enumerate vCPUs, and
  attribute guest page faults to process+VMA.

[cursor]

PR Summary

Medium Risk
GDB is feature-gated and restore-only override, but changes vCPU pause/resume and debug-event handling on multi-vCPU paths; default builds unaffected.

Overview
Adds GDB guest-kernel debugging on snapshot restore (x86_64, --features gdb): load-snapshot accepts optional gdb_socket_path (API + swagger); restore applies it via MachineConfigUpdate and build_microvm_from_snapshot wires vCPU debug channels and starts the GDB server at the saved RIP. Boot path gains resolve_gdb_socket_path (machine-config or FIRECRACKER_GDB_SOCKET).

All-stop multi-vCPU behavior: pause_all_vcpus on initial attach, each stop, and Ctrl-C; resume_all_vcpus drains stale queued debug events before resume to avoid multi-vCPU GdbQueueError under breakpoint storms.

snapshot-editor prints per-MSR index/data from vcpu state (KASLR slide from MSR_LSTAR). New integration tests cover restore+GDB (file/UFFD, 4K/2M) and fault attribution; test harness passes gdb_socket_path and skips post-resume SSH when GDB holds the guest.

Minor: copyright headers, doc/markdown wrapping, import/format churn in persist modules and tests.

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