Derive serde Serialize/Deserialize on Completeness

[mrpollo]

Completeness derives Clone, Debug, PartialEq, Eq but not serde::Serialize / Deserialize. Downstream tools that persist parse results (e.g. flight-review-rs writes a metadata.json) want to record how a log ended — complete, truncated, or malformed — but can't serialize the enum directly. They currently have to define a local mirror enum plus a From conversion just to get it into JSON, which duplicates the type and drifts if a variant is ever added.

Request

Derive (or feature-gate) serde::Serialize and Deserialize on Completeness so it can be persisted directly:

// src/stream_parser/model.rs:176
#[derive(Clone, Debug, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
pub enum Completeness {
    Complete,
    Truncated,
    MalformedRecord(String),
}

A sensible default wire form is snake_case externally-tagged, i.e. "complete" / "truncated" / { "malformed_record": "..." } — #[serde(rename_all = "snake_case")] gets that.

Notes

• If you'd rather not take an unconditional serde dependency, gate it behind a serde feature so consumers opt in.
• This only needs to cover Completeness. UlogParseError / ParseErrorType don't need it for this use case (the malformed string is already carried inside MalformedRecord).

Context

• px4-ulog 0.6.1, origin/master 3b27306, enum at src/stream_parser/model.rs:177
• consumer: Truncated ULog files produce no output (whole conversion aborts on a malformed tail) flight-review-rs#24 — will mirror the enum locally for now and switch to the parser's type once this lands
• follow-up to Truncated ULog files discard all parsed messages instead of returning the valid prefix #5 / Recover valid prefix from truncated logs instead of discarding it #6 / read_file_with_simple_callback discards truncated logs (streaming path missing the #6 recovery) #7