add-adapterlisted

# Adding a pond adapter An adapter is a bidirectional codec between one agent's native session format and pond's canonical schema (docs/spec.md section 6). The bar it must meet: lossless capture (every source field recoverable), no synthesized values, additive re-sync, and a restore face that is either lossless-native or an honest refusal. The registry cost is fixed and small: one file under `packages/pond/src/adapter/` plus its wiring in `src/adapter/mod.rs` - a module declaration, a re-export, and the `registry()` entry (spec 6.7). The work splits into two phases. Phase A produces the spec doc and the fixture; Phase B implements from them. Ship both in ONE self-contained PR: spec doc + fixture + adapter + tests + doc rows. Opening a draft PR after Phase A, so the decision table gets reviewed before implementation, is recommended but not required. Ground rules that override any instinct to improvise: - docs/spec.md section 6 is the contract. The named rules cited below (`adapter-integrity-*`, `adapter-bounded-values`, `model-no-synthesis`, ...) are binding, and each states WHY it exists - read the rule before deviating. - `cargo fmt --check`, `cargo clippy --all-targets -- -D warnings`, `cargo test` green is the whole bar for an adapter PR. No benchmarks: adapters are import-isolated from the store and query layer (a guard test in `src/adapter/mod.rs` enforces it and lists its exemptions with reasons), so an adapter cannot reach the store's write path, commit discipline,