perf(r): reuse single-chunk nanoarrow vectors

[paleolimbot]

Adds an as_nanoarrow_array.nanoarrow_vctr() fast path for full-slice, single-chunk vectors. Existing buffers are reused; a shallow array wrapper is created only when a data-frame field name must be attached. This avoids extension round-trips such as rebuilding GeoArrow geometry through WK.

On a 3.23M-row GeoArrow reproduction, data-frame-to-stream conversion fell from 3.44 s to 0.32 s. The available R test suite and R CMD check --no-manual pass.

library(nanoarrow)
x <- as_nanoarrow_vctr(as_nanoarrow_array(c("one", "two")))
as.data.frame(as_nanoarrow_array(data.frame(x = x)))
#>     x
#> 1 one
#> 2 two

[codecov-commenter]

Codecov Report

❌ Patch coverage is 92.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.21%. Comparing base (8f81815) to head (cc80c4f).

Files with missing lines	Patch %	Lines
r/R/vctr.R	92.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #951      +/-   ##
==========================================
+ Coverage   78.19%   78.21%   +0.02%     
==========================================
  Files         106      106              
  Lines       16845    16870      +25     
  Branches     1985     1985              
==========================================
+ Hits        13172    13195      +23     
- Misses       2439     2441       +2     
  Partials     1234     1234              

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.