WIP: fix(gnovm): validate assignment targets in range, compound assign, and inc/dec

[ltzmaxwell]

Depends on #5764 (stacked on its head; review the commits above the merge boundary).

• assertValidAssignLhs was only called from the plain-= branches, so unassignable targets slipped through elsewhere: for _, c = range a with const c and c++ panicked at runtime, for k, nil = range m passed preprocess (nil static type aliases blank), and c += 1 / s[0] += 1 ran with the write silently discarded.
• Fix: call assertValidAssignLhs on the non-blank range value operand, the compound-assign LHS, and the IncDecStmt operand — after the operator/type checks for the latter two, matching go/types error precedence (add_assign_a0, runtime_a3, shift_d44 goldens).
• Tests: types/assign_range_f/g/h.gno (const and nil as range value), types/assign_op_a/b.gno (const +=, string-index +=), types/incdec_a5.gno (const ++).

[Gno2D2]

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[ltzmaxwell]

Folded into #5764 — the range key/value handling and target validation are one design; reviewing them together is more coherent. All commits from this branch are merged there.