Unify the AstVisitorBase Stmt.Expression handler name (VisitExpressionStmt → VisitExpression)

[nickna]

Small naming cleanup surfaced while reviewing #1243 / PR #1249.

Every AST node's visitor handler follows the mechanical rule Visit + node type name (Stmt.Var→VisitVar, Expr.Binary→VisitBinary, …). Stmt.Expression is the one exception, and only in the analyzer-visitor family:

Family	Handler for Stmt.Expression
Analyzer visitors (AstVisitorBase + subclasses)	VisitExpressionStmt
Registry dispatch (Interpreter, TypeChecker, via NodeRegistry.AutoRegister's $"Visit{Name}")	VisitExpression

So the same node type has two different handler names depending on the subsystem. It's the only node where the two families disagree.

Not a bug — each family is internally consistent (AstVisitorBase's switch calls VisitExpressionStmt and that method exists; AutoRegister looks for VisitExpression on the interpreter/type-checker and those exist). The Stmt suffix was likely chosen to avoid the read-ambiguity of "VisitExpression" looking like "visit an Expr."

Why unify: it's the last divergence from the otherwise-uniform Visit{Name} convention, and it's the one thing a future source-generator-based dispatch (the road not taken in #1243) would have to special-case. Unifying removes that wrinkle.

Scope — rename VisitExpressionStmt → VisitExpression, 4 internal spots (behavior-neutral, near-zero risk; AstVisitorBase is not a public package API):

1. Parsing/Visitors/AstVisitorBase.cs — the Stmt.Expression switch case (~:94)
2. Parsing/Visitors/AstVisitorBase.cs — the method definition (~:434)
3. Compilation/StringAccumulatorPromotionAnalyzer.cs — the override (~:111)
4. Compilation/StringAccumulatorPromotionAnalyzer.cs — the base.VisitExpressionStmt(stmt) call (~:127)

Only argument against: slight readability loss ("VisitExpression" reading as an Expr visit).

Part of #1094.