feat(rtl): add hb_HGetDef and PValue / hb_PValue

Two standard Harbour functions that fivenode-style PRG code (bridge_*.prg
and downstream apps) calls frequently. Without them, every reference
emits an analyzer WARN and resolves to NIL at runtime.

* hb_HGetDef(hHash, xKey, xDefault) — hash lookup with fallback.
* PValue(nIndex[, xDefault]) — read the nth parameter of the calling
  PRG function. Mirrors the PCount pattern: needs the caller frame's
  paramCount and locals, exposed via new hbrt.Thread.CallerLocal helper
  that pairs with the existing CallerParamCount.

Registered under PVALUE and HB_PVALUE (Harbour accepts both forms).

Verified: hb_HGetDef / PValue / HB_PVALUE all return expected values for
present-key, missing-key-with-default, missing-key-no-default, and
out-of-range-param cases. Full regression: go test (18 packages) +
Compat 56/56 + std.ch 17/17 + FRB 7/7 + FiveSql2 43/43 all green.

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

hbrt/thread.go

@@ -670,6 +670,19 @@ func (t *Thread) CallerParamCount() int {
 	return 0
 }
 
+// CallerLocal returns the n-th parameter of the calling PRG function
+// (1-based). Returns NIL if no caller frame exists or n is out of range.
+// Pairs with CallerParamCount for implementing the PValue() RTL.
+func (t *Thread) CallerLocal(n int) Value {
+	if t.callSP >= 2 {
+		caller := &t.calls[t.callSP-2]
+		if n >= 1 && n <= caller.paramCount {
+			return caller.GetLocal(n, t.locals)
+		}
+	}
+	return MakeNil()
+}
+
 // PendingParams2 sets pending param count for direct block calls (AEval, ASort etc.)
 func (t *Thread) PendingParams2(n int) {
 	t.pendingParams = n