Phase 3: Scanner restructure + FieldSink trait #269

Summary

Rename ScanBuilder → FieldSink. Ensure the scan loop works in no_std core. Add bounded Kani proof and proptest oracle for scan_line.

Tasks

• Rename ScanBuilder trait to FieldSink in core
• Verify scan_into and scan_line compile under no_std
• logfwd-arrow builders implement FieldSink
• Add Kani bounded proof for scan_line (≤128 byte lines, ≤8 fields)
• Add proptest oracle: scan_line output vs serde_json parse
• Verify existing scanner_conformance tests still pass through re-exports
• Optional: add secondary type-based API (ParsedLine) for testing convenience

FieldSink trait

pub trait FieldSink {
    fn begin_batch(&mut self);
    fn begin_row(&mut self);
    fn end_row(&mut self);
    fn resolve_field(&mut self, key: &[u8]) -> usize;
    fn field_str(&mut self, idx: usize, value: &[u8]);
    fn field_int(&mut self, idx: usize, value: &[u8]);
    fn field_float(&mut self, idx: usize, value: &[u8]);
    fn field_null(&mut self, idx: usize);
    fn field_raw(&mut self, line: &[u8]);
}

Assignability

Needs us for trait design and Kani proofs. Copilot can handle the rename/mechanical refactoring once the trait is defined.

Parent: #262