[Stephen Belanger (Qard)]

Both ingestion upload paths previously grouped rows by row bytes alone, omitting the JSON envelope and separators. They also made separate decisions about oversized payloads and HTTP 413 responses.

Introduce a generic ordered batcher, a shared logs3 encoder, and a pure upload planner. Batch limits now include the complete request body; exact-limit bodies fit. Oversized rows remain explicit singleton work for overflow or a terminal size error. A rejected direct request is replanned as overflow or ordered splits, never retried unchanged. Both existing upload paths use these components; protocol and retry consolidation follows in the next layer.

Validation: 230 unit/integration tests and 7 doctests passed (existing ignores unchanged), strict clippy, rustdoc, formatting, and diff checks passed. Added coverage for envelope/separator/UTF-8 sizes, overflow-safe weight arithmetic, oversized rows, metadata, ordered 413 splits, and 413-to-overflow fallback.

Stacked above #118. Planning documents remain outside version control.