Refactor naming for message flow

[LiGoldragon]

Motivation

• Adopt context-minimal, object-first naming to remove redundant semantic noise in types and crates.
• Replace encoding- and example-specific identifiers with a generic Message/Text domain to separate representation from identity.
• Provide concise, direction-oriented Cap’n Proto backend APIs (from/to) that match the object-first naming rules.
• Update orchestration and tests to use the new names so the workspace remains consistent.

Description

• Renamed the application crate and binary from show_hello_world_from_stdin to show_from_stdin and updated workspace membership by editing Cargo.toml and Cargo.lock.
• Replaced the Cap’n Proto schema hello_world.capnp with message.capnp and changed the generated include to message_capnp.rs in build.rs and lib.rs.
• Refactored domain types in crates/types from HelloWorldObject/HelloWorldUtf8TextObject to Message/Text and updated their Sajban projection codec implementations accordingly.
• Simplified the Cap’n Proto backend API by renaming CapnpPackedProjectionBackendObject to CapnpPackedProjectionBackend, read_object_from_stdin_as_packed_capnp to from_stdin, write_object_to_packed_capnp_bytes to to_bytes, and the error object to CapnpPackedProjectionBackendError, and updated all call sites.
• Updated tests and test harness (text_projection_round_trip.rs) to encode/expect the new Message schema and to invoke the renamed application crate.

Testing

• No automated tests were executed as part of this change.
• Source references were updated across the workspace to avoid stale symbol usage.

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Codex Task