Add backup JSONL output and fix terminal rendering

[tijs]

Summary

• add attic backup --json as a schema-versioned JSON Lines progress stream with exactly one final completion event
• keep JSON stdout free of ANSI, filenames, paths, raw provider errors, migration text, hook output, and other human diagnostics
• preserve backup exit behavior and success/failure hooks while routing hook stdout to stderr in JSON mode
• serialize terminal callback rendering into immutable, width-aware frames; track actual frame height, clear stale rows, and make failure/cancellation cleanup idempotent
• leave stdin terminal modes unchanged so SIGINT/SIGTERM cannot strand the terminal in raw/no-echo mode
• document the stable event/count/bytes/pause/concurrency/final-result contract and compatibility with status --json and human output

Related to #13. The issue is intentionally left open for validation.

Verification

• swift test --filter BackupJSONProgressTests — 7 tests passed
• swift test --filter BackupCommandFlowTests — 6 tests passed
• swift test --filter TerminalRendererTests — 9 tests passed
• swift test — 323 tests in 49 suites passed
• swift build -c release — passed
• swiftformat . --lint — passed; 0 files require formatting
• swiftlint lint --quiet — exit 0; 9 warnings, identical to the base commit
• git diff --check — passed
• release CLI backup --help — verified --json usage and help text
• bounded PTY harness — normal redraw path emitted 64 ANSI sequences, handled a long filename, and restored terminal state
• bounded SIGINT PTY harness — process exited on SIGINT with ECHO/ICANON unchanged