Support transfers for layouts with multiple id-indexed fields

[2dm]

This patch adds a fast‐path for multi‐field copies when source and destination instances store fields consecutively in memory with strictly increasing IDs. Instead of the generic per‐field iterator, Realm switches to an IDIndexedIterator and issues the entire copy as a single batched DMA operation.

Fast‐Path Mechanics

• Layout Detection: During instance construction, Realm identifies when field IDs form a consecutive sequence and marks the instances for the optimized path.
• IDIndexedIterator: Attaches a single FieldBlock (allocated from the replicated heap) to the address list, skipping per‐field offset lookups.
• Affine Collapse: Treats the multi‐field operation as one affine rectangle with N attached fields, collapsing what would be N separate copies into a single batched command.
• Flow‐Control Splitting: The DMA engine automatically splits the batch according to internal flow‐control limits and IB fragment sizes (for cross‐node transfers).

Testing

• Integration Test: Verifies end-to-end multi-field copy behavior on real instances.
• Unit Tests: Covers iterator construction, address-list setup, FieldBlock allocation, and DMA splitting logic.

src_inst{ 0: [..field_size..], 1: [..field_size..], 2: [..field_size..], N-1: [..field_size..] }
dst_inst{ 0: [..field_size..], 1: [..field_size..], 2: [..field_size..], N-1: [..field_size..] }
index_space.copy(src_fids[32, 16, 612, 0..], dst_fids[17, 3, 999, 2..])

Authored by @apryakhin

[codecov]

Codecov Report

❌ Patch coverage is 39.81191% with 192 lines in your changes missing coverage. Please review.
✅ Project coverage is 29.20%. Comparing base (f337379) to head (30251c5).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/realm/transfer/transfer.cc	23.87%	160 Missing and 9 partials ⚠️
src/realm/inst_layout.inl	65.11%	15 Missing ⚠️
src/realm/runtime_impl.cc	0.00%	3 Missing ⚠️
src/realm/transfer/channel.h	0.00%	2 Missing ⚠️
tests/unit_tests/idindexed_fields_iterator_test.cc	95.83%	2 Missing ⚠️
src/realm/transfer/channel.cc	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #284      +/-   ##
==========================================
+ Coverage   29.07%   29.20%   +0.12%     
==========================================
  Files         194      195       +1     
  Lines       40229    40493     +264     
  Branches    14458    14693     +235     
==========================================
+ Hits        11697    11826     +129     
- Misses      27598    28242     +644     
+ Partials      934      425     -509     

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[apryakhin]

@2dm Thanks for adding this PR to the github. What's our strategy here? Are you proposing to carry it through the review process or do we plan to split it up?

[apryakhin]

@2dm The OG gitlab CI was passing for all tests (except clang-format):

• https://gitlab.com/StanfordLegion/legion/-/pipelines/1865577933

It's either some regression with the additional changes we make to reduce copy launch overhead or there are different settings/tests in github CI compared to gitlab.

[apryakhin]

overall LGTM, left just one comment