fix(gnovm): handle blank range key/value per-operand, validate assignment targets

[ltzmaxwell]

Follow-up to #5751 — main target: blank range key/value handled per-operand. Along the way it generalizes target/RHS validation to every assigning statement (absorbs #5804) and fixes adjacent issues found while unifying the pattern.

• Main fix: RangeStmt.AssertCompatible checks each operand independently via the new evalAssignLhsType helper (validity asked of every operand; type check skipped for blank), fixing the for _, v = range slice panic and the string-branch message printing kt for vt.
• Hardening: checkAssignableTo/assertIndexTypeIsInt panic on nil destination type — nil aliases an untyped-nil lvalue (for k, nil = range m) with blank, so a missing blank-skip now fails loudly in CI instead of silently skipping the check.
• Adjacent fixes: unassignable targets rejected at preprocess everywhere (for _, c = range a with const, c += 1 / s[0] += 1 silently discarding writes, c++ runtime panic — IncDecStmt.AssertCompatible now takes (store, last) like its peers); _ = nil rejected (T(nil) stays legal); compound assigns get blank-target (_ += 1) and RHS validity (i += int, i += f()) checks; range/index ,ok checks resolve baseOf and use assignability instead of kind, fixing silent type confusion with declared map types and named int/rune operands — all with go/types-matching messages and error precedence.
• Tests: types/range_blank_key2–5, assign_range_f–k, assign_index_d/e, assign_op_a–g (c–g also pin the check ORDER: each program has two errors and the golden records which fires first), incdec_a5, assign_nil3; nil-dt panic pinned in TestCheckAssignableTo.

[Gno2D2]

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[davd-gzl]

This also fix

package main

func main() {
	a := []int{1, 2, 3}
	var v int
	for _, v = range a {
	}
	_ = v
	println("ok")
}

// Output:
// ok

Ref: #5751 (comment) (+ verified personally)

[ltzmaxwell]

@davd-gzl requesting a fresh review: your approval was at fa5c11d3, and the PR has been substantially reworked since — it absorbed #5804 (closed) and now covers, beyond the original blank-range fix: a strict non-nil destination-type contract in checkAssignableTo (panics instead of vacuously accepting, with the nil-alias rationale in the description), assignment-target/RHS validation for range value, compound assign, and inc/dec, _ = nil rejection, and declared/named-type fixes (baseOf dispatch, assignability instead of kind checks for range index/value). Error messages and check ordering match go/types and are pinned by goldens (assign_op_c–g). Best reviewed as a fresh pass rather than a delta.