yson-rs convergence: offer the patches, measure the drift, agree a direction

[sshaplygin]

Part of #63. Human-gated.

ss123she/yson-rs is alive again: 0.1.3 to crates.io on 12 August 2026, 0.2.0 within days. crates/ytsaurus-yson carries a vendored and modified copy of it at revision ba2044c (v0.1.3), taken under Apache-2.0 as NOTICE records — so the fork and upstream are now moving at the same time, and every week apart is drift that has to be re-derived later.

• offer the three defect patches upstream
• diff the fork against upstream 0.2.0 and record what the drift actually is — until that exists, "we diverged" is a feeling
• have the direction conversation with the author before the two codebases separate permanently
• whatever is agreed: the vendoring paperwork in NOTICE stays intact, and this project does not claim the yson-rs name