fix(python): share per-object locks and align StartChild timeouts

[dbrattli]

Concurrent F# lock calls on the same object currently create separate Python RLock instances, allowing their critical sections to overlap. Async.StartChild also raises Python's built-in TimeoutError instead of .NET's TimeoutException when its deadline expires.

Changes

• Share a reentrant lock per object identity through a WeakValueDictionary. A short registry guard makes lookup and creation atomic and is released before acquiring the object lock or running user code. Holders and waiters retain their selected lock; unused entries are reclaimed without retaining the locked objects. Identity keys support unhashable and non-weak-referenceable objects.
• Use the existing start_child entry point for both F# and Python callers. Deadline expiry raises TimeoutException, whose native base from merged feat(python): give System exceptions native Python bases #5040 preserves Python except TimeoutError handlers. Remove the separate start_child_with_timeout entry point and compiler special case.
• Preserve the exact child exception object, including child-raised Python TimeoutError and .NET TimeoutException. Only cancellation caused by deadline expiry is translated.
• Exercise locking through generated F# tests covering exclusion, independent objects, reentrancy, return values, exception release, and storage reclamation. Add timeout and child-failure regressions in tests/Python/TestAsync.fs; await their completion so failures reach the runner. Remove the standalone timeout test and its build-copy step.

Validation

• Full repository Python gate with a freshly generated runtime: 2,403 .NET tests and 2,504 generated Python tests passed on Python 3.14.
• Full generated Pyright remains at the existing 34 errors / 45 warnings baseline.
• Restoring stripe-based identity mapping in an isolated process makes the independence regression fail. The original timeout handler also fails the child-exception identity probes.
• Source Fantomas, changed timeout runtime Ruff lint/format, commit hooks, and git diff --check passed. Repository Fantomas policy excludes tests/**.
• The Python run emitted the existing async timer-thread warning (Event loop is closed) and generated syntax warning.

[github-actions]

Python Type Checking Results (Pyright)

Metric	Value
Total errors	34
Files with errors	4
Excluded files	4
New errors	✅ No

Excluded files with errors (4 files)

These files have known type errors and are excluded from CI. Remove from pyrightconfig.ci.json as errors are fixed.

File	Errors	Status
temp/tests/Python/test_hash_set.py	18	Excluded
temp/tests/Python/test_applicative.py	12	Excluded
temp/tests/Python/test_nested_and_recursive_pattern.py	2	Excluded
temp/tests/Python/fable_modules/thoth_json_python/encode.py	2	Excluded