refactor: refactor logical decoders into "field decoders"

[westonpace]

Previously we had "logical page scheduler" and "physical page scheduler". There was no concept of a "column scheduler" or "field scheduler" like we have on the encode path. This was going to be a problem for pushdown and dictionary encodings. Both of these operate on a field-level (technically dictionary encoding might someday operate on both a page and field level but we can ignore that for now).

This PR converts the "logical page scheduler" concept into a "field scheduler" which spans entire columns.

The result is that the struct scheduler got simpler (some of its complexity moved to the primitive scheduler) and the list scheduler stayed more or less as complex as it has always been (but the work got split up a bit better). Performance-wise there should be no real difference but this opens up the floor for pushdown, dictionary, rle, etc.

There is also the small perk of aligning better with the encoder traits which have always had "field encoder"

[wjones127]

bikeshed: this could be Arc<[PageInfo]>, which eliminates one pointer hop, I believe.

https://www.reddit.com/r/rust/comments/14bpihv/using_arct_instead_of_vect/

[westonpace]

Cool trick to learn! I went ahead and converted a few different Arc<Vec<T>> structures in the lance_encoding crate.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.90909% with 56 lines in your changes are missing coverage. Please review.

Project coverage is 79.72%. Comparing base (ee37912) to head (cecf5c7).
Report is 1 commits behind head on main.

Files	Patch %	Lines
rust/lance-encoding/src/encodings/logical/list.rs	88.49%	18 Missing and 11 partials ⚠️
rust/lance-encoding/src/decoder.rs	88.39%	6 Missing and 7 partials ⚠️
...ust/lance-encoding/src/encodings/logical/struct.rs	92.13%	4 Missing and 3 partials ⚠️
.../lance-encoding/src/encodings/logical/primitive.rs	94.44%	4 Missing and 1 partial ⚠️
...ust/lance-encoding/src/encodings/logical/binary.rs	93.93%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2407      +/-   ##
==========================================
+ Coverage   79.60%   79.72%   +0.11%     
==========================================
  Files         206      205       -1     
  Lines       56627    56400     -227     
  Branches    56627    56400     -227     
==========================================
- Hits        45079    44963     -116     
+ Misses       8873     8756     -117     
- Partials     2675     2681       +6     

Flag	Coverage Δ
unittests	79.72% <90.90%> (+0.11%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.