3a: Add Attachment wire type to trapfall-proto

[ajianaz]

Parent: #233 · Blocked by: #235

Scope

Add Attachment struct to trapfall-proto/src/lib.rs and extend ParsedEnvelope.

Struct to Add

pub struct Attachment {
    pub filename: String,
    pub content_type: Option<String>,        // "image/png"
    pub attachment_type: Option<String>,     // "event.attachment" (default)
    pub data: Vec<u8>,                       // raw binary bytes
}

Extend ParsedEnvelope

pub struct ParsedEnvelope {
    pub events: Vec<Event>,
    pub transactions: Vec<Transaction>,
    pub session_updates: Vec<SessionUpdate>,
    pub session_aggregates: Vec<SessionAggregates>,
    pub attachments: Vec<Attachment>,         // NEW
}

Tasks

• Add Attachment struct
• Extend ParsedEnvelope with attachments field
• Unit test: create Attachment with binary data, verify fields
• Note: Attachment does NOT derive Serialize/Deserialize for the data field via serde_json — handle serialization separately (base64 for storage)

Effort: ~30 minutes