Implement ldiv!() for QRSparse

[JamesWrigley]

I originally was going to implement a separate workspace like how FastLapackInterface.jl does it, but seeing as there's already a QRSparse type that kinda functions as a workspace already I decided it made more sense to re-use that. Reusing QRSparse is also consistent with the way that UmfpackLU holds a workspace. Unlike UmfpackLU I did not add a lock because for the example in the workspace reuse test it added a ~15% overhead. Instead the docstring clearly warns that using QRSparse with ldiv!() is not threadsafe and tells users to make an explicit copy.

Preallocating W removed most of the allocations, and taking a view of F.R removed the other big one. Benchmarks:

using SparseArrays, LinearAlgebra

m, n = 100, 10
nn = 100

A = sparse([1:n; rand(1:m, nn - n)], [1:n; rand(1:n, nn - n)], randn(nn), m, n)
F = qr(A)
b = randn(m)
x = zeros(n);

# Before
julia> @benchmark $F \ $b                                                                                                                                                                                                                      
BenchmarkTools.Trial: 10000 samples with 17 evaluations per sample.                                                                                                                                                                            
 Range (min … max):  995.294 ns … 203.601 μs  ┊ GC (min … max): 0.00% … 98.64%                                                                                                                                                                 
 Time  (median):       1.245 μs               ┊ GC (median):    0.00%                                                                                                                                                                          
 Time  (mean ± σ):     1.324 μs ±   3.433 μs  ┊ GC (mean ± σ):  6.15% ±  2.59%                                                                                                                                                                 
                                                                                                                                                                                                                                               
  ▁▅▅▃▁   ▁▂▃▃▂  ▄▄▆█▇▆▅▄▂▁  ▁▁▁▁▂▁                             ▂                                                                                                                                                                              
  ██████▇██████████████████▇███████▇▆▃▅▄▄▄▂▂▃▄▅▂▅▅▇▇▆▇██▇▇███▇▆ █                                                                                                                                                                              
  995 ns        Histogram: log(frequency) by time       1.81 μs <                                                                                                                                                                              

 Memory estimate: 2.14 KiB, allocs estimate: 14.

# After
julia> @benchmark ldiv!($x, $F, $b)
BenchmarkTools.Trial: 10000 samples with 15 evaluations per sample.
 Range (min … max):  957.467 ns …   3.822 μs  ┊ GC (min … max): 0.00% … 0.00%
 Time  (median):       1.154 μs               ┊ GC (median):    0.00%
 Time  (mean ± σ):     1.148 μs ± 128.500 ns  ┊ GC (mean ± σ):  0.00% ± 0.00%

   ▃             █  ▁  ▅                                         
  ▄█▂▂▁▁▁▁▁▁▁▁▁▁▂█▅▃█▃▂█▆▂▆▄▁▂▂▁▁▂▁▁▁▂▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁ ▂
  957 ns           Histogram: frequency by time         1.61 μs <

 Memory estimate: 96 bytes, allocs estimate: 2.

Fixes #242. Written with help from Claude 🤖

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.23%. Comparing base (4500d86) to head (5a7c482).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #676      +/-   ##
==========================================
+ Coverage   84.19%   84.23%   +0.04%     
==========================================
  Files          12       12              
  Lines        9313     9339      +26     
==========================================
+ Hits         7841     7867      +26     
  Misses       1472     1472              

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[rayegun]

The asymmetry between UMFPACK and SPQR here on thread safety is concerning, but otherwise LGTM.

A 100 x 10 sparse matrix is incredibly small so I'm not sure the 15% hit here is really representative.