fix: forbid non-dollar names starting with $

[jodavies]

No description provided.

[coveralls]

coverage: 58.431% (+0.001%) from 58.43%
when pulling c982339 on jodavies:dollar-decl
into c549571 on form-dev:master.

[tueda]

Strangely, I got the following Valgrind error with

Local $test = 1;
.end

FORM 5.0.0 (Mar 31 2026, v5.0.0-11-gc982339)     Run: Fri Apr  3 17:19:53 2026
    Local $test = 1;
test.frm Line 1 --> Illegal name for expression
==636271== Invalid read of size 4
==636271==    at 0x404FF3D: DoExpr (comexpr.c:251)
==636271==    by 0x40507EF: CoLocal (comexpr.c:66)
==636271==    by 0x4066E94: CompileStatement (compiler.c:696)
==636271==    by 0x4112B5A: PreProcessor (pre.c:1130)
==636271==    by 0x4154E3D: main (startup.c:1819)
==636271==  Address 0x4 is not stack'd, malloc'd or (recently) free'd
==636271==

But the Valgrind check in CI passed...?

[jodavies]

Right, I can reproduce that locally. The same thing happens with, for eg,

Local test_ = 1;
.end

It looks like AC.ProtoType is only set properly in the non-error branch -- it would be straightforward to simply return the error immediately upon finding a bad expression name?