- Add ActiveChecksumContext enum to Tx to track and reuse context descriptors
- Add igc_tx_ctx_setup() to program the Advanced TX Context Descriptor for
  IPv4, TCP/IPv4, and UDP/IPv4 checksum offload
- Update igc_send() to prepend a context descriptor when offload context changes
- Advertise CSUM_IPv4, CSUM_TCPv4, and CSUM_UDPv4 capabilities
- Enable TCP checksum offload in if_net and update the capability comment

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Validated on real I225 hardware (UDP echo + TCP both work, valid on-wire
checksums confirmed via tcpdump on the peer).

- IP header length in the advanced context descriptor was programmed in
  32-bit words instead of bytes (`header_len()` returns IHL). With the wrong
  length the NIC mis-placed the checksums and IP packets never egressed
  correctly; shift IHL left by 2 to get bytes (matches the igb driver).
- Seed the IPv4 pseudo-header checksum into the L4 checksum field before DMA.
  smoltcp leaves it zero when TX checksum is offloaded, so without the seed
  the NIC computed segment-only checksums and every TCP/UDP checksum was wrong
  on the wire. Add `igc_pseudo_cksum()` and write it at the L4 csum offset.
- Restore `active_checksum_context` when `igc_send()` bails out due to
  insufficient descriptors, so the software context cannot desync from what
  the hardware actually saw (addresses the review's state-desync concern).
- Reclaim context descriptors in `igc_txeof()`: they carry no DD writeback,
  so the per-descriptor DD scan would stall at the first one and eventually
  wedge TX once the ring filled. Treat a non-DD descriptor whose successor is
  done as a consumed context descriptor.
- Derive `Copy, Clone` for `ActiveChecksumContext` to support the snapshot.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Validated on real I225 hardware (UDP echo + TCP, all egress checksums valid).

- igc_txeof: guard the context-descriptor look-ahead against the producer
  index so reclaim can never advance `next_to_clean` at/past `next_avail_desc`
  and corrupt `igc_desc_unused()` accounting.
- igc_send: reserve room for the worst case (ctx + data) with an up-front
  `igc_desc_unused() < 2` check (OpenBSD igc style), removing the fragile
  snapshot/restore of `active_checksum_context` on the bail-out path.
- igc_tx_ctx_setup:
  - fast-path early return when `csum_flags` is empty, skipping header parsing
    on packets that need no offload (e.g. ARP).
  - drop (InvalidPacket) when `extract_headers` fails on an offload-requested
    frame instead of posting an unchecksummed packet (mirrors igb/ixgbe).
  - drop IP fragments: the L4 header is not at the expected offset and hardware
    L4 offload cannot complete a fragmented checksum.
  - account for an inline 802.1Q VLAN tag (18-byte L2 header) when computing
    MACLEN and the pseudo-header seed offset.
- if_net::tx_packet_header_flags: gate TCP/UDP_CSUM_OUT on IPv4 so a non-IPv4
  (e.g. IPv6) TCP/UDP packet is not left with an unfilled checksum.
- Add IgcDriverErr::InvalidPacket.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>