fix #25978: show call chain for forbids violations

[nimamasl114514]

.forbids violations previously reported only the immediate call site with a misleading "template/generic instantiation from here" message, giving no indication of where the forbidden effect actually originates.

This PR:

• Replaces pushInfoContext (which produced the misleading template/generic message) with a direct ".forbids spec declared here" hint
• Adds traceForbidsOrigin which walks the callee's effect list to trace the effect back through the call graph, emitting a hint at each hop

Before:

test.nim(11, 25) template/generic instantiation from here
test.nim(12, 4) Error: c() has an illegal effect: NestedPoll

After:

test.nim(11, 25) Hint: .forbids spec declared here
test.nim(9, 14)  Hint: effect propagated from here
test.nim(8, 14)  Hint: effect propagated from here
test.nim(7, 17)  Hint: effect propagated from here
test.nim(4, 23)  Hint: effect propagated from here
test.nim(12, 4)  Error: c() has an illegal effect: NestedPoll

The raises path is unchanged — only isForbids takes the new code path.