fix typo in GC thrashing detection condition (#61112)
`thrash_counter` is initialized to `0` but only ever incremented under
this branch, which was checking `!(thrash_counter < 4)`, making the
entire mechanism effectively dead code
here was something that can demonstrate the difference but I don't know
how to add a unit test
```
# gc_thrash_mwe.jl
mutable struct Node; l::Any; r::Any; d::Vector{UInt8}; end
tree(n) = n <= 0 ? Node(nothing,nothing,rand(UInt8,64)) : Node(tree(n-1),tree(n-1),rand(UInt8,64))
const SHED = Ref(false)
const CB = @cfunction(() -> (SHED[] = true; nothing), Cvoid, ())
ccall(:jl_gc_set_cb_notify_gc_pressure, Cvoid, (Ptr{Cvoid}, Cint), CB, 1)
trees = Any[tree(18) for _ in 1:8] # ~640 MB scattered pointers
cache = Ref{Any}([rand(UInt8, 200) for _ in 1:1_000_000]) # ~200 MB droppable cache
GC.gc()
@time for i in 1:200
if SHED[]; cache[] = nothing; SHED[] = false; end
[rand(UInt8, 48) for _ in 1:50_000]
end
println(cache[] === nothing ? "PASS: cache shed under pressure" : "FAIL: pressure callback never fired")
```
```
➜ (adienes) ./julia --heap-size-hint=1500M gc_thrash_mwe.jl
0.449891 seconds (20.03 M allocations: 1.121 GiB, 64.03% gc time, 4.14% compilation time)
PASS: cache shed under pressure
➜ (adienes) julia +nightly --heap-size-hint=1500M gc_thrash_mwe.jl
0.619595 seconds (20.03 M allocations: 1.121 GiB, 75.61% gc time, 2.99% compilation time)
FAIL: pressure callback never fired
```
Discovered via automated Claude audit for typos and other minor /
obvious bugs.
