Linux SIGSEGV in idLexer::CreatePunctuationTable caused by signed char indexing

## Linux SIGSEGV in `idLexer::CreatePunctuationTable` caused by signed char indexing

### Description

OpenPREY consistently crashes during startup on CachyOS Linux while initializing declarations.

I tracked the crash with GDB to `src/idlib/Lexer.cpp`, specifically `idLexer::CreatePunctuationTable()`.

### Environment

* CachyOS x86_64
* GCC 16.2.1
* Meson 1.12.0
* Ninja 1.13.2
* OpenPREY main branch
* PREY 2006 Linux installation

### Crash

Startup reaches:

```text
----- Initializing Decls -----
Parsing Guides...
Found 0 guides...
```

and then crashes with SIGSEGV:

```text
Thread 1 "OpenPrey-client" received signal SIGSEGV, Segmentation fault.

idLexer::CreatePunctuationTable(
    this=0x7fffffffc9c0,
    punctuations=default_punctuations
)
at ../src/idlib/Lexer.cpp:550
```

Backtrace:

```text
#0  idLexer::CreatePunctuationTable()
    at ../src/idlib/Lexer.cpp:550
#1  idLexer::SetPunctuations()
    at ../src/idlib/Lexer.cpp:665
#2  idLexer::idLexer()
    at ../src/idlib/Lexer.cpp:2172
#3  idDeclFile::LoadAndParse()
    at ../src/framework/DeclManager.cpp:786
#4  idDeclManagerLocal::RegisterDeclFolder()
    at ../src/framework/DeclManager.cpp:1525
#5  idDeclManagerLocal::Init()
#6  idCommonLocal::InitGame()
#7  idCommonLocal::Init()
#8  main()
```

### Cause

At the crash, GDB showed:

```text
i = 52

newp->p = "�"

(unsigned char)newp->p[0] = 0xef
(int)newp->p[0] = -17
```

The code at line 550 uses:

```cpp
idLexer::punctuationtable[(unsigned int) newp->p[0]]
```

On this platform, `char` is signed. Therefore the byte `0xef` is interpreted as `-17` before being converted to `unsigned int`.

Instead of indexing the 256-entry table with:

```text
239
```

it effectively attempts to index it with:

```text
4294967279
```

resulting in the SIGSEGV.

There are three occurrences of this expression in `CreatePunctuationTable()`:

```text
Lexer.cpp:550
Lexer.cpp:558
Lexer.cpp:570
```

### Tested fix

Changing all three from:

```cpp
(unsigned int) newp->p[0]
```

to:

```cpp
(unsigned char) newp->p[0]
```

fixes the crash.

For example:

```cpp
for (n = idLexer::punctuationtable[(unsigned char) newp->p[0]];
     n >= 0;
     n = idLexer::nextpunctuation[n]) {
```

and the two assignments later in the function likewise use:

```cpp
idLexer::punctuationtable[(unsigned char) newp->p[0]] = i;
```

### Result

After rebuilding OpenPREY with those three changes, PREY successfully gets past declaration initialization and launches into the game.

The fix was tested on the same CachyOS/GCC 16.2.1 system that consistently reproduced the crash before the change.

I can submit the three-line change as a PR if desired.


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Linux SIGSEGV in idLexer::CreatePunctuationTable caused by signed char indexing #24