lib: cgen: replace counters map with per-CPU array

[qdeslandes]

Switch BF_MAP_TYPE_COUNTERS from BPF_MAP_TYPE_ARRAY to
BPF_MAP_TYPE_PERCPU_ARRAY. Each CPU updates its own counter slot
without contention, which eliminates the need for atomic operations
in the BPF fast path (bpf_map_lookup_elem on a per-CPU map returns
a direct pointer to the current CPU's value).

Changes

lib:

• Add BF_BPF_MAP_TYPE_PERCPU_ARRAY to bf_bpf_map_type
• Switch the counters map to BF_BPF_MAP_TYPE_PERCPU_ARRAY
• bf_handle_get_counter(): allocate a per-CPU buffer, do one lookup,
  then sum all CPU slots into the returned bf_counter
• Add bf_handle_set_counter(): builds a per-CPU buffer with the value
  in CPU 0's slot and zeroes elsewhere, then calls
  bf_bpf_map_update_elem() with the full buffer
• _bf_cgen_transfer_counters(): replace bf_map_set_elem() with
  bf_handle_set_counter() — the old call passed a 16-byte buffer to
  a syscall that reads num_cpus × 16 bytes, writing garbage from the
  stack into the new map

tests:

• Update bpftool map dump jq queries in 8 e2e tests: per-CPU dumps
  use values (array per CPU) instead of value, so counter reads
  change from .[N].value.count to
  .[N].values | map(.value.count) | add

[qdeslandes]

@claude review

[github-actions]

Claude review of PR #547 (f0720d2)

Suggestions

• Use backticks instead of @p in Doxygen — src/libbpfilter/cgen/handle.h:150 — ast-grep doxygen-prefer-backticks rule (severity: error) flags @p counter and @p counter_idx; style guide mandates backticks
• Repeated per-CPU buffer allocation in loop — src/libbpfilter/cgen/handle.c:402 — bf_handle_set_counter allocates/frees a per-CPU buffer on each call; in _bf_cgen_transfer_counters this happens N+2 times per chain update

CLAUDE.md improvements

• The style guide's @p prohibition could be referenced in CLAUDE.md's Comments section for visibility (currently only in doc/developers/style.rst)

Workflow run