ci: make CHANGELOG.md ready for ShipIt (#768)

* ci: pad historical versions to strict SemVer in CHANGELOG.md

ShipIt's changelog scanner parses every "## X.Y[.Z]..." heading with
SemVersionStyles.Strict, which requires MAJOR.MINOR.PATCH. The three
oldest entries (0.15, 0.14, 0.13) were two-part, so the first real
ShipIt run failed with:

    System.Exception: Invalid version
      at findVersions ... src/Generate/Changelog.fs:42

Rewrite those headings to 0.15.0 / 0.14.0 / 0.13.0. The content is
unchanged and the three-part form matches what PyPI would canonicalize
these to anyway.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* docs(changelog): reconstruct entries for 3.0.0, 4.0.0, 5.0.0a1, 5.0.0a2

Fill the gap between the existing 2.0.0-alpha entry and the upcoming
5.0.0-rc.1 release. Bullets are curated from git log between tag pairs
(v1.5.9..v3.0.0, v3.2.0..v4.0.0, v4.1.0..v5.0.0a1, v5.0.0a1..v5.0.0a2)
plus PR #743 for the 5.0.0a1 fluent-chaining headline — not exhaustive,
just the major changes (API rewrites, package rename, tooling moves,
Python-version floors).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* docs(changelog): use SemVer for 5.0.0 alpha headings

The historical 5.0.0a1 / 5.0.0a2 entries were written in PEP 440 form
(matching PyPI), but CHANGELOG.md headings must be strict SemVer for
ShipIt's parser — same reason we just padded 0.13/0.14/0.15. PEP 440
'5.0.0a1' fails SemVersionStyles.Strict; rewrite as 5.0.0-alpha.1 /
5.0.0-alpha.2, consistent with the preamble note that "ShipIt records
SemVer in this file" and with the upcoming 5.0.0-rc.1 heading.

The PEP 440 form still lives on PyPI (published wheel metadata) and in
pyproject.toml / reactivex/_version.py at release time, where the
shipit_bump_version.sh updater script performs the SemVer -> PEP 440
translation.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: dbrattli

CHANGELOG.md

@@ -54,6 +54,58 @@ this file; the release script normalizes between them.
   `Scheduler` base class.
 - CI: Standardised `actions/setup-python` to `@v5` across all workflow jobs.
 
+## 5.0.0-alpha.2
+
+- Documentation header formatting fixes.
+
+## 5.0.0-alpha.1
+
+- **Fluent method chaining** — operators are now available as methods on
+  `Observable` in addition to the existing `pipe()` style. Both work
+  interchangeably and can be mixed freely. Implemented via eleven category
+  mixins (transformation, filtering, combination, error handling,
+  windowing/buffering, multicasting, etc.). Zero breaking changes from 4.x —
+  all existing `pipe()`-based code continues to work. (#743)
+- Migrated project tooling from Poetry to `uv`.
+- Replaced Black + isort with Ruff for formatting and linting.
+- Dropped Python 3.8; added support for Python 3.11, 3.12, 3.13.
+- Added `scheduler` parameter to `run()` for consistent scheduling control.
+- Fixed `subscribe_on` not forwarding scheduler to the source. Thanks to
+  @MainRo
+- Minor internal: renamed `ReplaySubject.window` to `ReplaySubject._window`
+  to free the `window` name for the new method.
+
+## 4.0.0
+
+- Renamed the distribution from `rx` to `reactivex` (`pip install reactivex`).
+- Flattened the package: `Observable` and operators now live at the top level
+  (`rx.core` gone).
+- Raised minimum Python to 3.7.
+- Full pyright-strict type coverage; `disallow_untyped_defs` enforced.
+- Migrated from `setup.py` to Poetry.
+- Improved scheduler type annotations and asyncio thread-safety.
+- Internal operators renamed with a leading underscore (`_map`, `_filter`,
+  etc.).
+- Marble-diagram documentation added for every operator.
+- Test scheduler and marble-testing improvements. Thanks to @MainRo
+
+## 3.0.0
+
+- Replaced method chaining with a pipe-based functional API as the
+  recommended style.
+- Dropped Python 2 support; Python 3.6+ only.
+- Renamed the `concurrency` package to `scheduler`; split mainloop and
+  eventloop schedulers into their own modules.
+- Simplified variadic operator signatures (`amb`, `combine_latest`, `concat`,
+  `merge`, `on_error_resume_next`, `catch`, `zip`) — the leading `Iterable`
+  argument is gone.
+- Removed `result_mapper` from `join`, `group_join`, `zip`, `generate`.
+- Added comprehensive type hints and a `py.typed` marker (PEP 561).
+- Collapsed `AnonymousObservable` into `Observable`.
+- Renamed `catch_exception` to `catch`.
+- Added `TrampolineScheduler` and priority-queue scheduler improvements.
+- Replaced the blocking observable with a single `run()` function.
+
 ## 2.0.0-alpha
 
 - Extension methods and extension class methods have been removed. This
@@ -170,23 +222,23 @@ this file; the release script normalizes between them.
 - Refactored operators to use C# like extensionmethods using function decorators
 - More PEP8 alignment
 
-## 0.15
+## 0.15.0
 
 - Python slicing and indexing of observables. Thus you can write xs[1:-1:2]
 - Aligned backpressure with RxJS
 - Renamed all `select()` to `map()` and `where()` to `map()`
 - `from_` is now an alias for `from_iterable`. Removed `from_array`
 - Fixes for `flat_map`/`flat_map`. Selector may return iterable
 
-## 0.14
+## 0.14.0
 
 - Made `ScheduledObserver` thread safe
 - Thread safe handling for `take_while` and `group_join`
 - Removed dependecy on six (https://pythonhosted.org/six/)
 - Added support for IronPython (by removing six)
 - Aggregate is now an alias for reduce
 
-## 0.13
+## 0.13.0
 
 - Aligning throttle type operator naming with RxJS and RxJava
 - Added `throttle_last()` as alias for `sample()`