Document version 3.0.0 changes and migration

CHANGELOG.md

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+# Changelog
+
+## 3.0.0
+
+This release reworks logging ownership for Nim's ORC memory manager, fixes
+background writer shutdown, and switches generated timestamps to UTC. It
+introduces breaking API changes from 2.1.2. Nim 2.2.0 or later remains required.
+
+### Breaking changes and migration
+
+- **Service handles and local caches are separate types.** `LogService` is now
+  a copyable handle that keeps shared configuration alive. Pass a copy to each
+  worker, then call `threadLocalRef()` inside that worker. Keep
+  `ThreadLocalLogService` and its loggers in the thread that creates them. Use
+  `local.service` instead of `local[]` to obtain a shared handle.
+- **`ensureFreshness(LogService)` has been removed.** Refresh the relevant
+  `ThreadLocalLogService` instead. Configuration methods remain available on
+  both types; `LogService.appenders()` returns clones of the current shared
+  configuration.
+- **Custom appender callbacks now receive `state: var T`.**
+  `initCustomLogAppender` copies supported value state into each clone and
+  rejects ordinary references, closures, and embedded synchronization
+  primitives, including nested ones. Use `SharedLogState[T]` for shared mutable
+  value data, or `initFactoryCustomLogAppender` for independently created state
+  containing references or closures.
+- **Appenders now accept `LogFormatter` objects.** The former
+  `LogMessageFormatter` callback type is replaced by `StatefulLogFormatterFunc`
+  and `StatelessLogFormatterFunc`. Noncapturing formatter functions still
+  convert automatically. Replace directly supplied captured closures with
+  `initLogFormatter(seed, factory)`, and use `formatter.format(message)` to
+  invoke a formatter directly. Custom appender and formatter clones must own
+  independent state or use explicit shared-state handles.
+- **Generated timestamps now use UTC.** `LogMessage.ts` remains a `DateTime`;
+  structured JSON output uses a `Z` suffix. Explicit timestamps supplied in
+  caller-constructed messages are preserved.
+- **The test collector returns value snapshots.** `LoggedMessages.get()` now
+  returns `seq[RecordedLogMessage]` instead of `seq[LogMessage]`. Timestamps are
+  `Time` values, additional JSON data is serialized text, and exceptions retain
+  their name, message, and stacktrace as text. Use the collector's methods
+  instead of accessing its former `messages` field.
+
+### Added
+
+- `SharedLogState[T]` and `initSharedLogState` provide copyable ownership of
+  shared value data. Use `withState` for synchronized updates and `snapshot`
+  for independent copies. Raw pointers and custom copy/destruction hooks
+  remain the caller's responsibility.
+- `FactoryCustomLogAppender` and `initFactoryCustomLogAppender` create custom
+  state lazily within the thread using each clone. Formatter factories provide
+  the same ownership pattern for captured formatter state. Configuration
+  refresh can replace clones and reset their local state; use shared-state
+  handles for data that must persist across refreshes.
+
+### Fixed and clarified
+
+- Service copies retain shared configuration without sharing ordinary ORC
+  reference ownership between threads. Workers can outlive the original
+  handle and the thread that created it.
+- Local caches are built independently. Failed configuration refreshes preserve
+  the previous cache and retry on a later refresh without requiring another
+  configuration change.
+- Background console and file writers drain queued records at normal process
+  exit, including empty messages and the final file batch. Shutdown excludes
+  further submissions and waits for writers to finish. Applications must join
+  their logging workers before exiting.
+- Both message constructors avoid local-time conversion races by creating UTC
+  timestamps directly.
+- `isEnabled` intentionally checks cached namespace thresholds without
+  refreshing configuration. Logging calls, `getLogger`, or an explicit local
+  refresh pick up changes. Applications that guard all logging should refresh
+  at suitable boundaries, such as the start of a request. The shared root
+  threshold is read only when no cached namespace threshold applies.
+
+### Tests and known limitations
+
+- `nimble test` runs all enabled unit, regression, and ThreadSanitizer suites
+  under ORC with threads enabled. Individual suites remain available through
+  `testUnit`, `testRegression`, and `testThreadSafety`.
+- Validation passes 32 unit tests, 29 regression tests, and 11 sanitizer
+  scenarios. Two scenarios, `factory-state` and `allocator`, are temporarily
+  skipped because they expose a known race in Nim 2.2.10's default allocator.
+  Their code and assertions remain available; enable them with
+  `NSL_TEST_RUNTIME_ALLOCATOR=1 nimble testThreadSafety`.
+- The allocator limitation remains unresolved and also reproduces without
+  importing this library. Passing the enabled tests does not establish full
+  validation of factory state under the default allocator. See
+  [the test documentation](tests/README.md) for details and diagnostic results.
+
+See the [README](README.md) for usage and migration examples.