fix: PCount, Break/RECOVER, SET INDEX TO — 3 Harbour compat fixes

Release-blocking compatibility issues discovered during the 258-test
pre-release validation suite (100 syntax + 44 RDD + 114 RTL).

1. PCount() always returned 0 in PRG code

   Root cause: ParamCount() returned t.pendingParams, which is
   overwritten by every nested Function() call. By the time the
   PCount() RTL's Frame() executes, pendingParams is already 0.

   Fix: Frame() now stores pendingParams in frame.paramCount.
   PCount() RTL uses CallerParamCount() which reads callSP-2
   (the PRG caller's frame), while RTL functions still use
   ParamCount() (reads pendingParams before their own Frame).

   Verified: PCount(1,2,3)=3, PCount(1)=1, PCount()=0

2. Break("string") panicked instead of being caught by RECOVER USING

   Root cause: Generated SEQUENCE code only caught *HbError panics.
   Break() panics with BreakValue (a different type), which fell
   through to EndProc's "runtime error" message and re-panic.

   Fix (two parts):
   a) gengo emitBeginSequence: recover closure now catches any
      panic (interface{}), then dispatches via type switch:
      - *HbError → extract .Error() string
      - hasValue interface (BreakValue) → extract .GetValue()
      - other → static "error" string
   b) hbrtl/error.go: BreakValue gets GetValue() method for
      duck-type detection without import cycles
   c) hbrt/thread.go EndProc: BreakValue type name check added
      so it re-panics silently (no stderr noise)

3. SET INDEX TO a, b, c only opened the last file

   Root cause: Parser's parseSet() called parseExpr() once for
   INDEX setting, stopping at the first comma. Remaining file
   names were consumed by the "eat rest of line" loop.

   Fix: Parser now collects comma-separated identifiers into a
   single string literal "a,b,c". gengo splits on comma and
   calls OrderListAdd() for each file.

   Verified: SET INDEX TO si_name, si_city → OrdCount=2

All tests pass:
  go test ./...          14 packages OK
  FiveSql2               43/43  100%
  compat_harbour         51/51
  Syntax test           100/100
  RDD test               44/44
  RTL test              114/114
  Windows cross-compile  OK
  Linux cross-compile    OK

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

compiler/parser/parser.go

@@ -1781,6 +1781,35 @@ func (p *Parser) parseSet() *ast.SetCmd {
 		if upperSetting == "FILTER" || upperSetting == "RELATION" || upperSetting == "ORDER" || upperSetting == "INDEX" {
 			if p.current.Kind != token.NEWLINE && p.current.Kind != token.EOF {
 				expr = p.parseExpr()
+				// SET INDEX TO a, b, c — collect comma-separated file names
+				// into a single string literal "a,b,c" for gengo to split.
+				if upperSetting == "INDEX" {
+					getName := func(e ast.Expr) string {
+						switch v := e.(type) {
+						case *ast.IdentExpr:
+							return v.Name
+						case *ast.LiteralExpr:
+							return v.Value
+						default:
+							return ""
+						}
+					}
+					combined := getName(expr)
+					for p.current.Kind == token.COMMA {
+						p.advance()
+						if p.current.Kind != token.NEWLINE && p.current.Kind != token.EOF {
+							next := p.parseExpr()
+							combined += "," + getName(next)
+						}
+					}
+					if strings.Contains(combined, ",") {
+						expr = &ast.LiteralExpr{
+							Value:    combined,
+							Kind:     token.STRING,
+							ValuePos: expr.Pos(),
+						}
+					}
+				}
 			}
 			if p.current.Kind == token.INTO {
 				p.advance()