fix(pp,parser,gengo): pre-release blocker round (Wave 1)

Six audit-driven blockers landed together because they're tangled:

  * MENU TO removed from std.ch — the rule expanded to a call to a
    nonexistent __MenuTo() RTL symbol, so any user code with `MENU
    TO choice` compiled clean and panicked at runtime. Behavior
    pre-this-round was a parser silent no-op, which is at least
    consistent. Restore that until @ PROMPT (the companion command)
    actually lands.

  * COUNT now requires `TO <var>`. The earlier `[TO <v>]` optional
    bracket was a Harbour-pattern transcription error: the result
    template references `<v>` unconditionally, so a bare `COUNT`
    expanded to ungrammatical ` := 0 ; dbEval(...)` and the
    PRG parser rejected it. Match Harbour's std.ch which makes TO
    mandatory.

  * UPDATE FROM ... REPLACE now requires `FROM`/`ON`/`REPLACE` all
    three. Same root cause as COUNT: the result template uses
    `<key>`, `<f1>`, `<x1>` unconditionally; missing any of them
    produced broken syntax. Tightened to fail loudly rather than
    silently mis-expand.

  * CLOSE <unknown_alias> no longer closes the *current* workarea.
    SelectByAlias was a silent no-op when the alias was missing,
    leaving WASaveAndSelectAlias to evaluate the inner DbCloseArea()
    against the originally-selected WA — a real data-loss footgun.
    SelectByAlias now returns bool; WASaveAndSelectAlias switches to
    the no-area sentinel (0) on miss so the inner expression's
    Current() returns nil and short-circuits.

  * SUM <x1>, <xN> TO <v1>, <vN> — multi-pair form supported.
    Required two pieces:

       1. matchSegment's regular-marker stop-boundary now combines
          outerTail literals AND the segment's repeat boundary so
          `[, <xN>]` doesn't let `<xN>` swallow past the next ','.

       2. **Five parser miscompiled comma-separated expressions in
          code blocks.** `{|| e1, e2, e3 }` kept only the last expr
          and threw away earlier ones at *AST level*, so all their
          side effects vanished. New SeqExpr AST node + emitter
          (emit each, pop intermediate results) + folding/walk
          updates fix the underlying bug, which also unbreaks any
          other block that relied on comma sequencing.

  * pp.go's `;` continuation joiner now strips exactly one trailing
    `;` per iteration, preserving Harbour's `;;` convention (literal
    `;` followed by a continuation marker). Without this the SUM
    rule's chained `<v1> :=[ <vN> :=] 0 ; ; dbEval(...)` collapsed
    to a missing statement separator.

  * parseExprStmt's xBase fallback switch is back in sync with
    parseIdentStmt — COPY/SORT/COUNT/SUM/AVERAGE/TOTAL/UPDATE/JOIN/
    DISPLAY/LIST removed (std.ch handles all of them now). Leaving
    them in the fallback masked typos as silent no-ops.

Gates green:
  go test ./...      : PASS
  FiveSql2 SQL:1999  : 43/43
  Harbour compat     : 56/56

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

compiler/ast/ast.go

@@ -393,6 +393,22 @@ func (e *ArrayLitExpr) Pos() token.Position { return e.LBrace }
 func (e *ArrayLitExpr) End() token.Position { return e.RBrace }
 func (e *ArrayLitExpr) exprNode()           {}
 
+// SeqExpr is a comma-separated expression list used inside code
+// blocks: `{|p| e1, e2, e3 }`. All sub-expressions are evaluated in
+// order, the last value is the block's return. Without this node the
+// parser kept only the last expr and silently dropped the side
+// effects of every preceding one — a real miscompile that bit
+// `SUM x, y, z TO sx, sy, sz` (only sz accumulated).
+type SeqExpr struct {
+	Items   []Expr
+	StartAt token.Position
+	EndAt   token.Position
+}
+
+func (e *SeqExpr) Pos() token.Position { return e.StartAt }
+func (e *SeqExpr) End() token.Position { return e.EndAt }
+func (e *SeqExpr) exprNode()           {}
+
 // HashLitExpr represents a literal hash: {"a" => 1, "b" => 2}
 // Harbour: HB_ET_HASH
 type HashLitExpr struct {