feat(rwnd): add rwnd trace and its unit test

[zhang-hc22]

This PR solves issue #25

Description

This PR adds receive window (rwnd) trace support to netem-trace, enabling simulation and replay of TCP receiver-side behaviour over time.

Each step in a rwnd trace is described by four fields:

• duration — how long this configuration lasts before the next step applies
• set_rcv_buf (optional) — resize the socket's receive buffer to this value at this step
• app_read_bytes (optional) — simulate the application reading this many bytes from the receive buffer (drives the receiver model)
• rwnd_remaining (optional) — directly assert the observed remaining receive window after consumption (useful for replaying captured traces)

Exactly one of app_read_bytes / rwnd_remaining must be set per step, never both and never neither. This invariant is enforced at two levels: deserialization rejects invalid inputs with a clear error message, and StaticRwndConfig::build() panics for programmatically-constructed configs that omit both.

New public surface in src/lib.rs:

• Rwnd — type alias (u64) for all byte-count fields
• RwndAction — enum encoding the mutually-exclusive action per step (AppRead { bytes } / Remaining { rwnd })
• RwndDecision — a single step's payload (set_rcv_buf + RwndAction)
• RwndTrace — trait mirroring BwTrace/DelayTrace/LossTrace, returning Option<(RwndDecision, Duration)>

New model module src/model/rwnd.rs (enabled via rwnd-model / model feature):

• StaticRwnd / StaticRwndConfig — single-step model with builder API
• RepeatedRwndPattern / RepeatedRwndPatternConfig — repeating pattern over a sequence of steps, with infinite-loop (count = 0) support
• RwndTraceConfig — config-to-model conversion trait, typetag-registered for serde polymorphism
• Custom Serialize/Deserialize for StaticRwndConfig that flattens app_read_bytes/rwnd_remaining to the top level of the JSON object (consistent with the flat style of other models)

The rwnd-model feature is added to the model umbrella feature and follows the same dependency pattern as loss-model and duplicate-model (dep:dyn-clone).

How Has This Been Tested

• Unit tests (src/model/rwnd.rs #[cfg(test)] block):
  • test_static_rwnd_model_app_read — verifies AppRead step fields and exhaustion
  • test_static_rwnd_model_remaining — verifies Remaining step fields and exhaustion
  • test_repeated_rwnd_pattern — verifies a 2-step pattern repeated twice, then None
  • test_build_panics_without_action — confirms build() panics without an action set
  • test_serde_roundtrip_app_read — serialize → deserialize → assert field equality (#[cfg(feature = "serde")])
  • test_serde_roundtrip_remaining — same for Remaining variant
  • test_serde_rejects_both — confirms deserialization error when both fields are present
  • test_serde_rejects_neither — confirms deserialization error when neither field is present
• Doc tests on all public types and the module-level examples
• Tests run against --features "model,serde" (without human) and --features "model,serde,human" to confirm both duration-format paths work correctly. (The rejection tests were specifically written to avoid the "1s" human-format string to ensure they test the right thing regardless of features.)

cargo test --features "model,serde" # 22 unit + 47 doc — all pass
cargo test --features "model,serde,human" # 23 unit + 47 doc — all pass

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist

• Code follows the code style of this project.
• Changes follow the CONTRIBUTING guidelines.
• Update necessary documentation accordingly.
• Lint and tests pass locally with the changes.
• Check issues and pull requests first. You don't want to duplicate effort.

Other information

The RwndAction / RwndDecision types are placed in lib.rs alongside BwTrace, LossTrace, etc., matching twnd-model feature follows the same opt-in pattern as other model features and is included in the modelumbrella and full feature set.