refactor(ffi): 降级 FFI 为可选兼容层并整理文档

将 bat-ffi 明确收敛为无状态 C ABI 兼容层，默认集成路径改为 bat --json 进程边界或未来稳定 SDK。

同步 README、当前状态、项目计划、架构文档、开发指南和缺口清单，移除 FFI 作为主集成边界的表述。

新增 AGENTS.md 和 CONTRIBUTING.md，压缩 CLAUDE.md 为兼容入口，并归档阶段性报告、同步 .gitignore 规则。

crates/bat-ffi/Cargo.toml

@@ -9,20 +9,9 @@ license.workspace = true
 crate-type = ["cdylib", "staticlib"]
 
 [dependencies]
-bat-core = { path = "../../core" }
 bat-adapters = { path = "../../adapters" }
 bat-infrastructure = { path = "../../infrastructure" }
-bat-cas-engine = { path = "../bat-cas-engine" }
-bat-assetbundle = { path = "../bat-assetbundle" }
-bat-patch = { path = "../bat-patch" }
 
-anyhow.workspace = true
 serde.workspace = true
 serde_json.workspace = true
 tokio.workspace = true
-
-# FFI 绑定
-libc = "0.2"
-
-[build-dependencies]
-cbindgen = "0.27"