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>

hbrtl/error.go

@@ -103,6 +103,12 @@ type BreakValue struct {
 	Value hbrt.Value
 }
 
+// GetValue returns the Break() argument. Used by RECOVER USING via duck typing
+// (the generated code checks for a `hasValue` interface to avoid import cycles).
+func (bv BreakValue) GetValue() hbrt.Value {
+	return bv.Value
+}
+
 // Break(xValue) → panics with BreakValue, caught by BEGIN SEQUENCE/RECOVER.
 func Break(t *hbrt.Thread) {
 	nParams := t.ParamCount()

hbrtl/missing.go

@@ -269,11 +269,12 @@ func TypeFunc(t *hbrt.Thread) {
 	t.RetValue()
 }
 
-// PCount returns number of parameters passed.
+// PCount returns number of parameters passed to the calling PRG function.
+// Harbour: hb_pcount() — returns the CALLER's param count, not PCount's own.
 func PCount(t *hbrt.Thread) {
 	t.Frame(0, 0)
 	defer t.EndProcFast()
-	t.RetInt(int64(t.ParamCount()))
+	t.RetInt(int64(t.CallerParamCount()))
 }
 
 // Break moved to error.go — full implementation with BreakValue type.