fix(cpworker): keep the L2 header stack under zmq slice and count runt drops

[timmy21]

Problem

• A zmq output with slice < 18 dropped every packet. slice was applied first, then a fixed 18-byte minimum rejected the result. docs/ZMQ-WIRE-FORMAT.md said slice keeps at least Ethernet + one VLAN header.
• Dropped runt frames only incremented the internal nb_too_small_packets counter. No output statistic changed, so fwd + direction_drop + error_drop + ratelimit_drop did not add up to the packets the output handled.
• The 18-byte minimum was a leftover guard from before [cpworker] output_zmq.c: VLAN-aware slicing causes size_t underflow and out-of-bounds memcpy #231. It rejected valid untagged frames of 14–17 bytes, and still let slice cut into a QinQ tag stack (slice: 18 kept the outer tag and rewrote its EtherType to 0x8847).

Change

slice truncates the payload only; the Ethernet header and the whole VLAN tag stack are always kept.

• l2_header_len() measures Ethernet + complete tag stack on the original caplen, before slice is applied.
• Effective caplen with slice is max(slice, l2_len).
• A frame whose Ethernet header or tag stack was not fully captured produces no record and is counted in error_drop_packets / error_drop_bytes. The nb_too_small_packets warning stays and now names the reason.
• The second VLAN walk in zmq_send_packet is removed; the record reuses l2_len.
• docs/ZMQ-WIRE-FORMAT.md §1 and §4.1 describe the new rule.

Case	Before	After
slice: 10, one VLAN tag	dropped, not counted	18-byte record
slice: 18, QinQ	inner tag cut, outer EtherType rewritten	22-byte record, both tags
slice: 26, 4 tags (#231 frame)	26 bytes, 3 tags	30 bytes, 4 tags
untagged, 14–17 bytes	dropped, not counted	sent
< 14 bytes, or cut tag	dropped, not counted	dropped, error_drop

The two #231 tests are rewritten for the new rule. The 60-byte frame filled with 0x8100 tags has a tag stack that never ends within caplen, so it is now an error_drop instead of a record with 11 tags.

Tests

cpworker/tests/unit/output_zmq.c: slice keeps one tag / both QinQ tags / the 4-tag stack / the untagged header; 14-byte untagged frame sent; < 14 bytes, cut tag and unterminated tag stack counted as error_drop; a runt plus a normal frame gives fwd = 1, error_drop = 1.

• Unit tests: 9/9 suites pass.
• Integration tests: run_test.sh all passes.

Fixes #271