sysutils and pylib both declare Exception, and their shapes had diverged

Any program that pulled sysutils before pylib failed to compile with

error: undefined variable (msg)

reported at a line inside pylib, with nothing wrong there. It took out uses json in any Pascal program and examples/net/httpdemo.pas on every target (http.pas lists sysutils ahead of base64, and base64 pulls pylib).

Cause

Exception is deliberately shared program-wide (ClassNameIsDeliberatelyShared in symtab.inc — a NilPy except Exception: must catch what an RTL unit raises, so both declarations have to resolve to ONE row). Whichever unit registers first owns that row.

The two declarations were not interchangeable:

With pylib first everything worked. With sysutils first, pylib's own method bodies (constructor Exception.Create does msg := m) compiled against sysutils' class, which has no msg — hence the error, at a line the application never wrote.

Fix

sysutils' Exception now mirrors pylib's member for member: field msg first, FHelpContext second, FMessage/Message as properties over msg. Either registration order now yields a row both units' bodies compile against, and the field ORDER matches so the layout is the same whichever row wins. lib/rtl/json.pas and lib/rtl/pathlib.pas also list pylib first now, with a note saying why.

Keep the two declarations in step. The order-dependence itself is a design question tracked by [[decide-class-namespace-scoping]] — this ticket only makes the two shapes agree.