Add emit/parsing round-trip test cases and fix bugs found by them

.gitignore

@@ -3,3 +3,4 @@ target
 vendor/
 tags
 *.swp
+AGENTS.md

TODO.md

@@ -0,0 +1,65 @@
+# TODO
+
+## Bugs found with nlmon captured packets (fixed)
+
+- `EthtoolPauseStatAttr::Tx` emitted `ETHTOOL_A_PAUSE_STAT_RX_FRAMES`
+  instead of `ETHTOOL_A_PAUSE_STAT_TX_FRAMES` (`src/pause/attr.rs`).
+- `EthtoolLinkModeAttr::Ours`/`Peer`, `EthtoolTsInfoAttr` bitsets and
+  `EthtoolFecAttr::Modes`/`Stats` were emitted without the
+  `NLA_F_NESTED` flag the kernel sets.
+- Feature verbose bitset parsing dropped index 0 and name-less bits and
+  ignored `ETHTOOL_A_BITSET_SIZE`/`ETHTOOL_A_BITSET_NOMASK`, so the
+  kernel reply could not be emitted back byte for byte.
+- Compact bitsets (`ETHTOOL_FLAG_COMPACT_BITSETS`, used by `ethtool -k`)
+  failed to parse with "No ETHTOOL_A_BITSET_BITS NLA found".
+- FEC stats failed to parse when the driver has no corrected bits stats
+  (empty `ETHTOOL_A_FEC_STAT_CORR_BITS`) or reports per lane values.
+- Emitting parsed bitsets panicked with `todo!()` for feature, link mode,
+  timestamping and FEC messages.
+
+## unit tests
+
+- Module EEPROM (`ethtool -m`) reply parsing has no captured-packet
+  test. Requires SFP hardware which none of the virtual NICs
+  (virtio-net, veth, netdevsim) provide.
+- `EthtoolTsInfoAttr::TxTypes`/`RxFilters` parse paths are not covered,
+  virtio-net, veth and netdevsim do not report tx types/rx filters.
+- Non-empty `EthtoolLinkModeAttr::Ours`/`Peer` bitsets are not covered,
+  none of the available virtual NICs reports supported link modes.
+
+## API review
+
+- Review whether public types need `#[non_exhaustive]`. Candidates:
+  `EthtoolCmd` (kernel keeps adding commands), `EthtoolBitsetValue`
+  (two UAPI forms today). Attribute enums already have
+  `Other(DefaultNla)` catch-alls. `EthtoolBit`/`EthtoolBitset` have
+  public fields users need to construct for set requests, adding
+  `#[non_exhaustive]` there would block struct literals.
+
+## attributes kept as `DefaultNla`
+
+- `link_mode`: `ETHTOOL_A_LINKMODES_RATE_MATCHING` (10).
+- `rings`: `ETHTOOL_A_RINGS_RX_PUSH` (14),
+  `ETHTOOL_A_RINGS_TX_PUSH_BUF_LEN` (15),
+  `ETHTOOL_A_RINGS_TX_PUSH_BUF_LEN_MAX` (16),
+  `ETHTOOL_A_RINGS_HDS_THRESH` (17),
+  `ETHTOOL_A_RINGS_HDS_THRESH_MAX` (18).
+- `pause`: `ETHTOOL_A_PAUSE_STATS_SRC` (6) and
+  `ETHTOOL_A_PAUSE_STAT_TX_PAUSE_STORM_EVENTS` (4).
+- `tsinfo`: `ETHTOOL_A_TSINFO_HWTSTAMP_PROVIDER` (7),
+  `ETHTOOL_A_TSINFO_HWTSTAMP_SOURCE` (8),
+  `ETHTOOL_A_TSINFO_HWTSTAMP_PHYINDEX` (9).
+- `EthtoolModuleEEPROMAttr` only parses `HEADER` and `DATA`,
+  `OFFSET`/`LENGTH`/`PAGE`/`BANK`/`I2C_ADDRESS` fall through to
+  `DefaultNla` (they are only used in requests today).
+
+## missing support
+
+- `EthtoolMessage` parses a subset of ethtool commands only: `STATS_GET`,
+  `MODULE_GET`, `EEE_GET`, `PHY_GET`, `TSCONFIG_GET`, ... return
+  "Unsupported ethtool reply command".
+- Compact bitsets keep the raw `value`/`mask` words, there is no bit
+  name table to resolve the enabled features from a compact reply.
+- Unknown attributes nested inside a bitset are dropped with a warning,
+  a later kernel addition would break byte-exact round-trip of parsed
+  messages.