Give each error class one identity across subpaths and accept piped input in Rust (0.8.0)

[0thernet]

Summary

Wave B (B6) carry-over from Wave A's local-custody gaps.

• One error identity across subpaths. Each subpath used to bundle its own copy of ControlSocketError, ProtectedInputError and the custody errors, so error instanceof ControlSocketError failed when the class came from the root and the error from /control-socket. The build now uses Bun's code splitting (chunk-[hash].js in dist), so each class exists once. Determinism still holds across absolute roots (13 artifacts), and the packed-consumer smoke asserts class equality and a cross-subpath instanceof.
• Pipe input in Rust. read_protected_descriptor accepted regular files only, so pbpaste | <rust-cli> login --stdin failed with kind. It now accepts pipes and sockets like the TypeScript reader. Regular files keep the owner and no-group/other checks; directories and devices are still refused (kind). The spec says the same.
• Sidecar guard. The sidecar's stdio carries its protocol, and a pipe is now readable, so read_protected_descriptor with fd 0–2 and read_protected_stdin are refused with kind. That matches the old observable result; new spec vectors cover both. The TypeScript engine never sends either request.
• Version 0.8.0 (package, crate, inventory). README notes both changes.

Test plan

• bun run check (inventory, error copy, lint, typecheck, 72 tests, Rust fmt/clippy/test, build, Rust artifacts, determinism, packed smoke)
• New Rust tests: piped content, piped over-bound → limit, empty pipe → "", directory → kind
• New vector: sidecar read_protected_stdin → kind

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