hugolib/doctree: Simplify lock setup in SimpleThreadSafeTree to reduce read allocation

hugolib/doctree: Simplify lock setup in SimpleThreadSafeTree to reduce read allocation

```
                                      │    sec/op    │   sec/op     vs base               │
SimpleThreadSafeTree/Get-10             45.80n ± 11%   27.71n ± 7%  -39.51% (p=0.002 n=6)
SimpleThreadSafeTree/Insert-10          40.96n ±  1%   40.36n ± 0%   -1.47% (p=0.002 n=6)
SimpleThreadSafeTree/LongestPrefix-10   47.12n ±  0%   32.60n ± 1%  -30.81% (p=0.002 n=6)
geomean                                 44.54n         33.16n       -25.57%

                                      │ cmp152.bench │       perf-allloc20251118.bench        │
                                      │     B/op     │    B/op     vs base                    │
SimpleThreadSafeTree/Get-10               16.00 ± 0%    0.00 ± 0%  -100.00% (p=0.002 n=6)
SimpleThreadSafeTree/Insert-10            8.000 ± 0%   8.000 ± 0%         ~ (p=1.000 n=6) ¹
SimpleThreadSafeTree/LongestPrefix-10     16.00 ± 0%    0.00 ± 0%  -100.00% (p=0.002 n=6)
````