feat: add getAssignVarNames API to complement getOutVarNames (#464)

[chenjunwenhao]

Summary

Add Express4Runner.getAssignVarNames(String script) — a new introspection API that returns the set of context variables assigned (created or updated) by a script. This is the natural counterpart to the existing getOutVarNames(): while getOutVarNames identifies external inputs, getAssignVarNames identifies outputs/side-effects.

Behavior

Script	getAssignVarNames	getOutVarNames
a = b	{a}	{b}
object a = b	{} (typed decl = local)	{b}
a = 1; b += 1	{a, b}	{}
a = 1; b = a + 1	{a, b}	{}

Key design decisions:

• Typed declarations (int a = 10, String name = "hello") are excluded — they create locally-scoped variables, not assignments to the execution context.
• For-each loop variables (for(i : list)) are excluded — also locally-scoped.
• Increment/decrement operators (a++, --b) are included — they modify the variable.
• Field assignments (obj.field = value) do not include the base variable — only the field is modified.

Implementation

Introduces AssignVarNamesVisitor extending ScopeStackVisitor, following the same AST-walking pattern as the existing OutVarNamesVisitor and OutVarAttrsVisitor. The visitor collects LHS variable names from assignment expressions and handles suffix/prefix ++/-- operators.

Test plan

• Simple assignment (a = b → {a})
• Typed declaration excluded (object a = b → {})
• Compound assignment (b += 1 → {b})
• Multiple assignments
• Self-assignment (a = 1; a = a + 1 → {a})
• Complementarity with getOutVarNames on same script
• For-each loop variable excluded
• If-else / while scoping
• Field assignment (obj.field = 10 → {})
• Function parameters excluded
• Switch with typed declarations
• Selector variable assignment (${0} = ${1})
• Increment/decrement in sub-expressions (b = a++ + 1 → {a, b})
• No assignments (a + b, hello() → {})

Closes #464