Add chapters 26-30: Async, Multiprocessing, CLI, DateTime, Data Formats

[leandro-mana]

Summary

• Chapter 26: Async Programming — coroutines, event loop, asyncio.run(), create_task, gather, semaphores, async iterators/generators/context managers, producer-consumer
• Chapter 27: Multiprocessing and Parallelism — Process, Pool, Queue, Pipe, Value, Array, Manager, locks, ProcessPoolExecutor, CPU-bound vs I/O-bound
• Chapter 28: Command-Line Interfaces — argparse positional/optional args, types, choices, nargs, subparsers, mutually exclusive groups, os.environ, sys.argv, exit codes
• Chapter 29: Date, Time, and Scheduling — date, time, datetime, timedelta arithmetic, zoneinfo, aware vs naive, timezone conversion, strftime/strptime, ISO 8601, calendar
• Chapter 30: XML, HTML, and Data Formats — xml.etree.ElementTree, XPath, html.parser, html.escape, configparser, tomllib, CSV dialects, struct packing, plistlib

Each chapter includes 3 Jupyter notebooks, a README, and a comprehensive test file (63 new tests, 243 total).

Test plan

• ruff check . — all checks passed
• ruff format --check . — 55 files already formatted
• mypy . — no issues found in 28 source files
• pytest — 243 tests passed in 0.75s

Note: This PR is based on main before PR #6 was merged. It may need a rebase after PR #6 is merged.

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