bug fixes, speed improvements, improve shell support

[rrotter]

This is a bit of a catch-all. I kept each commit discrete by topic, but there are several topics covered in this PR. Each commit should apply cleanly in any order, so if you don't like all of them, feel free to mix and match. Or, if you prefer, I could resubmit them as more than one PR.

faster

hyperfine results:

macos	before	after	percent
dash	42.0 ms ± 1.1 ms	35.1 ms ± 0.6 ms	16%
bash	44.3 ms ± 0.5 ms	36.9 ms ± 0.5 ms	16%
zsh	45.9 ms ± 1.0 ms	38.7 ms ± 0.8 ms	16%

debian 12	before	after	percent
dash	6.4 ms ± 0.9 ms	4.2 ms ± 0.6 ms	34%
bash	8.2 ms ± 1.0 ms	5.2 ms ± 0.7 ms	36%
zsh	8.6 ms ± 1.0 ms	5.6 ms ± 0.8 ms	35%

fixed compatibility with zsh, osh, mksh

before:

NerdFetch % zsh nerdfetch
expr: not a decimal number: '53562.25'
expr: syntax error

      ___     rrotter@mond
     (.. \      macOS 15.5
     (<> |      24.5.0
    //  \ \   󰍛  53562.25/65536 MiB (%)
   ( |  | /|  󰏔  104 (brew)
  _/\ __)/_)  󰅶  10 days, 6 hours, 53 mins
  \/-____\/     ██████████████████
NerdFetch % osh nerdfetch
getprop: not found
  283583.
        ^
./nerdfetch:1: Unexpected token while parsing arithmetic: '.'
  283583.
        ^
./nerdfetch:1: fatal: Parse error in recursive arithmetic
NerdFetch % mksh nerdfetch
expr: division by zero
expr: syntax error

      ___     rrotter@mond
     (.. \      macOS 15.5
     (<> |      24.5.0
    //  \ \   󰍛  367/0 MiB (%)
   ( |  | /|  󰏔  104 (brew)
  _/\ __)/_)  󰅶  10 days, 6 hours, 53 mins
  \/-____\/     ██████████████████

after:

NerdFetch % zsh nerdfetch; osh nerdfetch; mksh nerdfetch

      ___     rrotter@mond
     (.. \      macOS 15.5
     (<> |      24.5.0
    //  \ \   󰍛  53832/65536 MiB (82)%
   ( |  | /|  󰏔  104 (brew)
  _/\ __)/_)  󰅶  10 days, 7 hours, 33 mins
  \/-____\/     ██████████████████

      ___     rrotter@mond
     (.. \      macOS 15.5
     (<> |      24.5.0
    //  \ \   󰍛  53847/65536 MiB (82)%
   ( |  | /|  󰏔  104 (brew)
  _/\ __)/_)  󰅶  10 days, 7 hours, 33 mins
  \/-____\/     ██████████████████

      ___     rrotter@mond
     (.. \      macOS 15.5
     (<> |      24.5.0
    //  \ \   󰍛  53832/65536 MiB (82)%
   ( |  | /|  󰏔  104 (brew)
  _/\ __)/_)  󰅶  10 days, 7 hours, 33 mins
  \/-____\/     ██████████████████

[ThatOneCalculator]

Thank you so much! Because there are a lot of individual changes, I will take the time to test this manually on my own machines with different configurations just to ensure that everything works as expected, so I may hold off on merging this until tomorrow or Monday – but when I do, this will likely mark 8.4.0.

[ThatOneCalculator]

Reviewed in multiple setups, hitting this with a good ol' LGTM!