Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@docs/snakefiles/rules.rst`:
- Around line 3133-3137: The example uses Python's builtin open() for Snakemake
file objects (variables like a, b in the loop over series) which bypasses
Snakemake's storage-aware IO; change these calls to use the Snakemake file
object's .open() method (e.g., a.open(), b.open()) so reads use IOFile.open()
and work with remote/non-shared storage, keeping the same read/strip/max logic
for val2 and val.

In `@src/snakemake/checkpoints.py`:
- Around line 59-83: The current __enter__/__exit__ replaces self._local.cache
causing nested checkpoint blocks to lose the outer cache; change the semantics
so __enter__ pushes a new list onto a stack (or wraps the existing cache in a
list-of-lists) instead of overwriting, and __exit__ pops the inner cache and, if
a parent cache exists, extend/merge the popped cache into the parent rather than
discarding it; only when popping the outermost cache and exc_type is None should
you combine exceptions (set e.extra = _e) and raise the
IncompleteCheckpointException. Ensure you update uses of self._local.cache in
checkpoints.rule*.get paths to work with the stack shape and restore the
previous cache on all exit paths.

In `@src/snakemake/exceptions.py`:
- Around line 595-601: The property targetfile currently returns a list,
breaking callers that expect a single path; change the property targetfile on
the exception class to return the singular path (self._targetfile) and add a new
property named targetfiles that returns the aggregated list (including extra
entries) so batched logic can use it; then update the batched input-expansion
branch in Rule.expand_params() to reference exception.targetfiles instead of
exception.targetfile while leaving single-path checks using exception.targetfile
intact.

Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@src/snakemake/checkpoints.py`:
- Around line 78-86: The __exit__ method references an undefined local variable
cache; change the logic to read the stored exceptions from self._local.cache
(e.g., cache = self._local.cache), check that cache is truthy when exc_type is
None, and then aggregate/raise as before (set e.extra = _e and raise e). Ensure
you read self._local.cache before you clear/reset it and still decrement/reset
self._local.depth correctly in the __exit__ implementation so the exception
aggregation using e.extra uses the stored cache instead of an undefined name.

Verify each finding against the current code and only fix it if needed.

Nitpick comments:
In `@src/snakemake/checkpoints.py`:
- Line 76: Replace the Chinese inline comment after the "return self" statement
in src/snakemake/checkpoints.py with an English comment for consistency; for
example change "不再需要 as cp" to "no longer need 'as cp'" (locate the comment
adjacent to the return self statement in the relevant method/class in
checkpoints.py).
- Around line 117-118: The return type on expect is incorrect: replace the
misleading union NoReturn | CheckpointJob with just CheckpointJob on the expect
method so it correctly reflects that subclasses (e.g., CheckpointLater) may
return a CheckpointJob; update the signature of expect(self, output) ->
CheckpointJob and ensure any necessary imports or forward references remain
valid (the method still raises IncompleteCheckpointException here, but the
declared type must be CheckpointJob).
- Line 83: Remove the redundant reset of self._local.depth to 0: since
_local.depth is decremented earlier (the decrement at the block that sets depth
to 0) and this branch is only entered when it is already zero, delete the
assignment "self._local.depth = 0" to avoid pointless code; update any nearby
comment if present in the same method that references this reset (look for the
decrement and the branch using self._local.depth in the same function/method).