Introduce a layered architecture separating protocol logic from I/O:

- Protocol layer (caldav/protocol/): Pure functions for XML building
  and parsing with typed request/response dataclasses (DAVRequest,
  DAVResponse, PropfindResult, CalendarQueryResult)
- Operations layer (caldav/operations/): Sans-I/O business logic for
  CalDAV operations -- properties, search, calendar management,
  calendar objects, principal discovery
- Response layer (caldav/response.py): BaseDAVResponse with shared
  XML response parsing for sync and async clients
- Data state (caldav/datastate.py): Strategy pattern for managing
  calendar data representations (raw string, icalendar, vobject)

This enables code reuse between sync and async implementations and
improves testability through pure function isolation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

- Add AsyncDAVClient with httpx (preferred) and niquests async fallback
- Extract BaseDAVClient ABC with shared auth, config, and URL logic
- Extract auth utilities to caldav/lib/auth.py (WWW-Authenticate
  parsing, auth scheme selection)
- Add caldav/aio.py as async entry point with backward-compat
  Async* aliases (AsyncCalendar, AsyncEvent, etc.)
- Refactor DAVClient to inherit from BaseDAVClient, reducing
  duplication between sync and async code paths
- Replace eager imports in caldav/__init__.py with PEP 562 lazy
  loading for faster startup (heavy deps deferred until first use)
- Remove caldav/objects.py backward-compatibility shim

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

- Make Calendar, Principal, Event, Todo, Journal work transparently
  with both DAVClient (sync) and AsyncDAVClient (async)
- Add get_* method aliases for server-fetching operations
  (get_calendars, get_events, get_todos, get_display_name, etc.)
- Deprecate date_search() in favor of search(), DAVObject.name in
  favor of get_display_name()
- Refactor search.py to generator-based Sans-I/O pattern --
  _search_impl yields (SearchAction, data) tuples consumed by
  sync or async wrappers
- Add CalendarObjectResource.id property returning UID
- Integrate DataState for efficient data representation management
- Add edit_icalendar_instance() and edit_vobject_instance() context
  managers for safe mutable access to calendar data

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

- Add get_connection_params() for unified config discovery from
  multiple sources (explicit params, env vars, config files)
- Add resolve_features() for feature profile resolution with
  base+override support in YAML config
- Raise errors on config file parse failures (breaking change)
- Add feature configuration validation
- Extend compatibility hints for Zimbra, Bedework, CCS, Davis,
  DAViCal, GMX, ecloud, Synology, SOGo, Posteo, PurelyMail,
  Xandikos, Baikal, and Radicale
- Fix _derive_from_subfeatures partial-config derivation bugs
- Add URL space validation in lib/url.py
- Fall back to principal URL when calendar-home-set is missing
- Minor code quality improvements in elements/ and lib/ modules
  (Python 3.10+ syntax, import cleanup, ruff formatting)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Replace legacy tests/conf.py with new tests/test_servers/ framework:

- Add TestServer base class hierarchy (Embedded, Docker, External)
- Add YAML-based server configuration (caldav_test_servers.yaml)
- Add ServerRegistry for server discovery and lifecycle management
- Add client_context() and has_test_servers() helpers
- Add Docker test infrastructure for CCS (Apple CalendarServer),
  DAViCal, Davis, and Zimbra
- Update existing Baikal, Cyrus, Nextcloud, SOGo Docker configs
- Add convert_conf_private.py migration tool for old config format
- Add fixture_helpers.py with shared test fixtures
- Remove deprecated tests/conf.py, conf_baikal.py, conf_private.py

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

New test suites:
- test_protocol.py: Unit tests for Sans-I/O protocol layer
  (XML building/parsing with typed dataclasses)
- test_operations_*.py (6 files): Unit tests for operations layer
  (properties, search, calendar, calendarobject, calendarset, principal)
- test_async_davclient.py: AsyncDAVClient unit tests
- test_async_integration.py: Async integration tests with test servers
- test_lazy_import.py: PEP 562 lazy import verification

Updated existing tests:
- test_caldav.py: Modernize integration tests for new test framework
- test_caldav_unit.py: Expand unit test coverage
- test_compatibility_hints.py: Add feature derivation tests
- Other test files: Import cleanup and ruff formatting

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

- Add comprehensive v3.0 code review (docs/design/V3_CODE_REVIEW.md)
- Add design documents for Sans-I/O architecture, API analysis,
  data representation, method generation, and implementation plans
- Add code review notes for niquests and urllib3-future dependencies
- Add async usage documentation (docs/source/async.rst)
- Add HTTP library configuration docs (docs/source/http-libraries.rst)
- Update tutorial, howtos, and about pages for v3.0
- Add async usage examples and get_calendars example
- Update all examples to use get_davclient() factory
- Expand CHANGELOG.md with comprehensive v3.0.0a1 entry covering
  lazy imports, context managers, feature profiles, compatibility
  hints, new test infrastructure, and all bug fixes
- Update README.md with async support highlights

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Zimbra does not support substring text matching - it accepts the
REPORT with text-match match-type="contains" but only does exact
matching. Searching for "Bastille Day" returns 0 results while
"Bastille Day Party" (exact summary) returns 1.

With this hint, the library correctly expects 0 results from
implicit substring searches and uses client-side filtering for
explicit substring searches.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Zimbra:
- Remove save-load.get-by-url unsupported hint (works in zcs-foss:latest,
  was broken in older Zimbra versions)
- Remove event_by_url_is_broken old flag (same reason)
- Add save.duplicate-uid.cross-calendar unsupported (Zimbra treats
  same-UID events across calendars as aliases of the same event)

Cyrus:
- Add save.duplicate-uid.cross-calendar unsupported (enforces unique
  UIDs across all calendars for a user)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Cyrus Docker container does not enforce password authentication,
accepting any password. Use the sentinel value that the test suite
already recognizes to skip the wrong-password test.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Replace conf_private.py references in Docker test server READMEs,
about.rst, test_caldav.py header, and RELEASE-HOWTO.md with the
new caldav_test_servers.yaml config format and TEST_<SERVER> env vars.

Also exclude caldav_test_servers.yaml and tmp_caldav_test_servers.yaml
from MANIFEST.in to prevent test configs from being included in PyPI
releases.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

…may have been a false negative due to their caching issue

compatibility_hints.py:
- Posteo: add search.time-range.todo.old-dates unsupported (recent dates
  work, old dates don't), add search.text.substring unknown
- Purelymail: mark flapping features as fragile (search.is-not-defined
  was full now ungraceful, search.time-range.event was unsupported now
  ungraceful, search.time-range.todo was ungraceful now unsupported),
  add search.text.substring unknown

search_ops.py:
- Catch ValueError from icalendar_searcher.check_component() when
  server returns data with invalid recurrence structure (e.g. after
  compatibility hacks strip DURATION). Include the object unfiltered
  rather than crashing. Fixes jeanes testRecurringDateWithExceptionSearch.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

I realize that I've broken my own AI-policy mutliple times now on other projects, and I will continue spending 30s on asking Claude to submit bug-fixing pull requests when needed  - so I'm changing the AI-policy

…f it's a bit slow