bug: group_by_dynamic period and every get applied in reverse order

[MarcoGorelli]

Checks

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of Polars.

Reproducible example

In [18]: df = pl.DataFrame({'a': [datetime(2020, 1, 1), datetime(2020, 2, 1), datetime(2020, 3, 1)], 'b': [1,2,3]})

In [19]: df.group_by_dynamic('a', every='1mo', period='45d', include_boundaries=True).agg(pl.col('b').sum())
Out[19]:
shape: (3, 4)
┌─────────────────────┬─────────────────────┬─────────────────────┬─────┐
│ _lower_boundary     ┆ _upper_boundary     ┆ a                   ┆ b   │
│ ---                 ┆ ---                 ┆ ---                 ┆ --- │
│ datetime[μs]        ┆ datetime[μs]        ┆ datetime[μs]        ┆ i64 │
╞═════════════════════╪═════════════════════╪═════════════════════╪═════╡
│ 2020-01-01 00:00:00 ┆ 2020-02-15 00:00:00 ┆ 2020-01-01 00:00:00 ┆ 3   │
│ 2020-02-01 00:00:00 ┆ 2020-03-15 00:00:00 ┆ 2020-02-01 00:00:00 ┆ 5   │
│ 2020-03-01 00:00:00 ┆ 2020-04-15 00:00:00 ┆ 2020-03-01 00:00:00 ┆ 3   │
└─────────────────────┴─────────────────────┴─────────────────────┴─────┘

Log output

No response

Issue description

It looks like period is getting applied before every, as opposed to

[start + every, start + every + period)

which is what's documented

I think the documentation expresses the desired behaviour - if you pass period=45d, then you expect each period to be 45 days long

Expected behavior

shape: (3, 4)
┌─────────────────────┬─────────────────────┬─────────────────────┬─────┐
│ _lower_boundary     ┆ _upper_boundary     ┆ a                   ┆ b   │
│ ---                 ┆ ---                 ┆ ---                 ┆ --- │
│ datetime[μs]        ┆ datetime[μs]        ┆ datetime[μs]        ┆ i64 │
╞═════════════════════╪═════════════════════╪═════════════════════╪═════╡
│ 2020-01-01 00:00:00 ┆ 2020-02-15 00:00:00 ┆ 2020-01-01 00:00:00 ┆ 3   │
│ 2020-02-01 00:00:00 ┆ 2022-03-18 00:00:00 ┆ 2020-02-01 00:00:00 ┆ 5   │
│ 2020-03-01 00:00:00 ┆ 2022-04-15 00:00:00 ┆ 2020-03-01 00:00:00 ┆ 3   │
└─────────────────────┴─────────────────────┴─────────────────────┴─────┘

Installed versions

--------Version info---------
Polars:              1.12.0
Index type:          UInt32
Platform:            Linux-5.15.153.1-microsoft-standard-WSL2-x86_64-with-glibc2.35
Python:              3.12.5 (main, Aug 14 2024, 05:08:31) [Clang 18.1.8 ]
LTS CPU:             False

----Optional dependencies----
adbc_driver_manager  <not installed>
altair               5.4.1
cloudpickle          3.0.0
connectorx           <not installed>
deltalake            <not installed>
fastexcel            <not installed>
fsspec               2024.9.0
gevent               <not installed>
great_tables         0.11.0
matplotlib           3.9.2
nest_asyncio         1.6.0
numpy                2.1.3
openpyxl             <not installed>
pandas               2.2.3
pyarrow              18.0.0
pydantic             <not installed>
pyiceberg            <not installed>
sqlalchemy           <not installed>
torch                <not installed>
xlsx2csv             <not installed>
xlsxwriter           <not installed>