perf: Improve join performance for new-streaming engine

[orlp]

This changes our representation from many pre-partitioned small dataframes on the build side to unpartitioned morsels + partition indexes, after which we do a precise reserve for the payload size and build one contiguous payload chunk per partition. This reduces a lot of pressure on the allocator, and is much more sensible for view types keeping references into the build-side payload buffer without endless deduping or memcpy'ing.

[codecov]

Codecov Report

Attention: Patch coverage is 36.17300% with 487 lines in your changes missing coverage. Please review.

Project coverage is 80.49%. Comparing base (40c7019) to head (0abac7b).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-stream/src/nodes/joins/equi_join.rs	42.12%	272 Missing ⚠️
...es/polars-core/src/chunked_array/object/builder.rs	0.00%	75 Missing ⚠️
crates/polars-expr/src/hash_keys.rs	9.09%	40 Missing ⚠️
crates/polars-arrow/src/bitmap/builder.rs	0.00%	39 Missing ⚠️
crates/polars-expr/src/idx_table/row_encoded.rs	44.61%	36 Missing ⚠️
crates/polars-utils/src/sparse_init_vec.rs	74.07%	14 Missing ⚠️
crates/polars-arrow/src/array/primitive/builder.rs	0.00%	6 Missing ⚠️
...s/polars-core/src/chunked_array/object/registry.rs	0.00%	3 Missing ⚠️
crates/polars-core/src/series/builder.rs	60.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #21620      +/-   ##
==========================================
- Coverage   80.56%   80.49%   -0.08%     
==========================================
  Files        1604     1604              
  Lines      231642   231678      +36     
  Branches     2650     2650              
==========================================
- Hits       186615   186481     -134     
- Misses      44410    44580     +170     
  Partials      617      617              

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