rust-cli: standalone cocoindex CLI for Rust projects

[tomz-alt]

Part of #2273. Spec: dev/plans/rust-cli.md (lands with the first PR).

One Rust binary with the same commands as the Python CLI. Pipeline logic is compiled into the
user's binary, so the CLI works like cargo run: build the crate, run the user's binary,
pass the subcommand through over versioned JSON-lines on stdio. ls --db / show skip the
build and read the state db directly via core::inspect.

• SDK registration: #[cocoindex::app] + #[cocoindex::lifespan] via linkme (same
  mechanism as #[cocoindex::function]), #[cocoindex::main] dispatch,
  Environment::from_registered(), EnvironmentBuilder::on_shutdown — the CLI path and
  the user's own main initialize environments through the identical lifespans
  (Rust analogue of @coco.lifespan + import)
• SDK: cli::try_dispatch() handling __coco ls|update|drop + protocol events
  (start / progress / preview / done / error), exit codes, version handshake
• new crate rust/cli/: ls [--db], show, update [-L] [--full-reprocess] [--preview] [--reset], drop [--db-only], init scaffold, --env-file
• migrate examples/rust/text_embedding as the validating example (cocoindex update,
  -L, --preview, show, drop all work; cargo run -- query untouched)

Phase 2 (follow-up issue once this lands): ship the binary in the Python wheels (ruff-style)
and replace the click front-end with a thin python -m cocoindex protocol child — one CLI
for both SDKs.