feat: add record subcommand for session recording

[gadenbuie]

Summary

Adds a shinyloadtest record subcommand that captures a user's interaction with a Shiny app into a recording file for later replay with shinyloadtest replay.

The recording system works as a reverse proxy that sits between the browser and the Shiny app, intercepting HTTP and WebSocket traffic. Key components:

• HTTP reverse proxy — forwards requests to the target app, captures initial GET as REQ_HOME
• WebSocket proxy — relays messages between client and server, recording WS_OPEN, WS_RECV, WS_SEND, and WS_CLOSE events with relative timestamps
• Token replacement — detects and tokenizes session-specific values (session ID, worker ID, Shiny tokens) so recordings are replayable across sessions
• Recording writer — streams events to a .log file in the existing shinycannon recording format
• Terminal UI — shows live event count, connection status, timestamps, and a quit hint via ora spinners
• Graceful shutdown — tracks active WS connections and defers shutdown by 500ms to handle brief connection overlaps

Also refactors replay-specific modules (output.ts, session.ts, ui.ts, websocket.ts, worker.ts) into src/replay/ to cleanly separate concerns.

Verification

# Record a session against a running Shiny app
npx @posit-dev/shinyloadtest record http://localhost:3838

# Interact with the app in the browser at the printed proxy URL,
# then press Ctrl+C to stop. A .log recording file is created.

# Replay the recording
npx @posit-dev/shinyloadtest replay recording.log http://localhost:3838

Tests: 6 new test files with ~1450 lines covering events, tokens, writer, HTTP proxy, WS proxy, and E2E lifecycle.