Stack space overflow RTS error misreports stack size
## Summary
The error message that the RTS produces when the stack overflows misreports the actual size of the stack.
## Steps to reproduce
Take the following program:
```hs
module Main (main) where
fib :: Integer -> Integer
fib 0 = 0
fib 1 = 1
fib n = fib (n-1) + fib (n-2)
main :: IO ()
main = print $ fib (10^6)
```
Then compile and run it like so:
```
$ ghc-9.12.2 Bug.hs -fforce-recomp -rtsopts
$ ./Bug +RTS -K100000 -RTS
Bug: Stack space overflow: current size 33608 bytes.
Bug: Use `+RTS -Ksize -RTS' to increase it.
```
Despite the fact that I configured the stack to have a size of 100,000 bytes, it reports that the actual size is 33,608 bytes.
## Expected behavior
I would expect that actual stack size to be closer to 100,000 bytes.
## Environment
* GHC version used: I reproduced this with GHC 9.4.8, 9.12.2, and 9.14.1-alpha1.
Optional:
* Operating System: I reproduced this with x86-64 Ubuntu 24.04 and ARM64 macOS 15.6.1.
issue