TUI walking skeleton (Phase 4)

First implementation milestone: Cargo project, dependencies,
and a minimal but functional TUI shell built on Ratatui +
Crossterm + Tokio in the Elm-style update/view pattern
(Candidate A from Phase 2/3 selection).

Includes:
- Three-region layout: items list (left), output + input + hint
  (right), bottom status bar with mode-aware shortcuts.
- Two themes (light, dark) plus COLORFGBG auto-detect, per
  NFR-7. CLI: --theme {light,dark}, --log-file <path>.
- Input modes per ADR-0003: simple (default), advanced, with
  the `:` one-shot escape including immediate prompt reaction
  ("Advanced:" label, advanced border) and auto-inserted space
  after a leading `:` in simple mode.
- App-level commands: `quit`/`q`, `mode simple`/`mode advanced`
  (canonical list per ADR-0003 — remaining commands land in
  later iterations).
- File logging via tracing, defaulting to ~/.rdbms-playground/
  playground.log so the TUI is not corrupted by stdio.

Testing per ADR-0008:
- Tier 1: 29 unit tests covering input handling, mode switch,
  one-shot escape, auto-space, output buffering, CLI parsing.
- Tier 2: 4 insta snapshots (default simple/advanced/light,
  one-shot active) of TestBackend frames.
- Tier 3: 7 integration tests driving synthetic events through
  App::update + render path.

All green: 36 tests, 0 failures, 0 skips. Clippy clean with
nursery lints enabled.

src/main.rs

@@ -0,0 +1,36 @@
+use std::process::ExitCode;
+
+use rdbms_playground::cli::Args;
+use rdbms_playground::{logging, runtime};
+
+fn main() -> ExitCode {
+    let args = match Args::from_env() {
+        Ok(args) => args,
+        Err(e) => {
+            eprintln!("rdbms-playground: {e}");
+            return ExitCode::from(2);
+        }
+    };
+
+    if let Err(e) = logging::init(args.log_path.as_deref()) {
+        eprintln!("rdbms-playground: failed to initialise logging: {e:#}");
+        return ExitCode::FAILURE;
+    }
+
+    let tokio_rt = match tokio::runtime::Runtime::new() {
+        Ok(rt) => rt,
+        Err(e) => {
+            tracing::error!(error = %e, "failed to start tokio runtime");
+            return ExitCode::FAILURE;
+        }
+    };
+
+    match tokio_rt.block_on(runtime::run(args.theme)) {
+        Ok(()) => ExitCode::SUCCESS,
+        Err(e) => {
+            tracing::error!(error = %e, "runtime exited with error");
+            eprintln!("rdbms-playground: {e:#}");
+            ExitCode::FAILURE
+        }
+    }
+}