fix(cuda): bound dictionary gathers by the dictionary length - #10915

…eset (#10917)

## Summary

- Tracking Issue: #10914
- Depends on #10915 and #10916; this branch contains both. The diff of
its own is one line plus a test.

`CompressionMode::Cuda` excludes `StringDictScheme`, but
`execute_dict_varbinview` already decodes string dictionaries on the
GPU. Keeping the scheme makes GPU-mode files 16.0% smaller across
compress-bench's 17-dataset GPU suite (3,351.3 → 2,813.9 MB).
Per-dataset gains: CMSprovider 45.7%, Food 52.9%, HashTags 33.2%, both
TPC-H l_comment variants 12.2%, Euro2016 10.7%. The issue has the full
table.

## Changes

- `vortex-btrblocks/src/builder.rs`: drops `StringDictScheme` from the
Cuda exclusions and updates the comment. A new test checks that the
preset keeps string dictionaries but not binary ones. This is the only
change to CPU-compressor code.

Evidence with this branch (A40, CUDA 13.0):
- `--gpu-verify` passes on all 17 datasets. develop plus only this line
fails 5: taxi, CMSprovider, Euro2016, Food, HashTags.
- The CPU-fallback census is identical to develop's: 59 constant-array
fallbacks, none from dictionaries.
- The dictionary gather takes at most 6.1% of decode time and reaches
62-85% of the measured 563 GB/s.
- `vortex-cuda` lib suite on this branch: 723 passed, 0 failed.
- `compress_gpu` fuzz (CI flags, ASAN): 100,000 runs in 256 s, 0
crashes.

## Benchmarks

Setup:
- `compress-bench --gpu-decompress --iterations 5`, a fresh process per
run, `taskset -c 2-7`, 3 ABBA rounds.
- Speedup = develop time / branch time.
- The same-design A/A floor is 0.14 in |log|, so a "faster" claim needs
a 95% CI above 1 and a speedup above 1.32×.

| Dataset | File MB, develop → branch | Speedup [95% CI] | |
|---|---|---:|---|
| Food | 84.7 → 39.9 | 1.55× [1.41, 1.71] | faster |
| CMSprovider | 771.0 → 418.5 | 1.37× [0.66, 2.86] | within noise |
| Euro2016 | 163.3 → 145.9 | 1.46× [0.92, 2.32] | within noise |
| TPC-H l_comment chunked | 169.6 → 148.8 | 1.36× [0.55, 3.34] | within
noise |
| taxi | 52.0 → 51.5 | 1.28× [0.32, 5.11] | within noise |
| HashTags | 235.2 → 157.0 | 1.22× [0.89, 1.69] | within noise |
| TPC-H l_comment canonical | 169.6 → 148.8 | 1.18× [0.71, 1.96] |
within noise |
| Arade | 144.2 → 141.7 | 1.05× [0.42, 2.68] | within noise |

The nine datasets whose files don't change got one AB/BA round each.
Eight came out at 1.01-1.18×, and one (wide table cols=1000 chunks=50)
at 0.91×, inside the A/A floor. The measurements are from an A40
(GA102); the repo's GPU benchmarks run on A10G.

## API changes

`only_cuda_compatible()`, and therefore `cuda_write_strategy`, can now
emit string dictionaries.
- An `ArrowDeviceArrayStream` with the default
`DictionaryExport::Preserve` rejects a batch whose Arrow schema differs
from the first.
- With this change, 6 of CMSprovider's 14 string columns mix dictionary
and constant chunks.
- Today's Cuda files already do this in numeric columns: 7 of taxi's 18
numeric columns and 5 of CMSprovider's 12.
- Streams with `DictionaryExport::Decode`, which the FFI scan uses, are
unaffected.

Overlap: #10425 adds a line next to the removed one, so whichever merges
second needs a one-line rebase. #9762 changes dictionary selection, so
the sizes are as of a59294e.

## AI assistance

Developed with Claude Code (Claude Opus 5.5), which wrote most of the
code, tests and this description under my direction. I reviewed the diff
and the measurements.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Signed-off-by: Tanish Gudise <[email protected]>
Co-authored-by: Claude Opus 5.5 <[email protected]>
Co-authored-by: Alexander Droste <[email protected]>