perf(FiveSql2): PcOpFieldTrim fused peephole — string WHERE at raw RDD parity

Second pcode peephole to match the one added for FieldGet(literal).
SqlExprToPrg auto-wraps CHAR column references with AllTrim() to
match SqlCmpEq's CHAR-padding trim semantics, so every string WHERE
predicate evaluates `AllTrim(FieldGet(n)) == 'literal'` per row.

Before this commit each of those per-row evaluations did:
  1. PushSymbol ALLTRIM
  2. PushSymbol FIELDGET → Function(1)  [1 RTL Frame]
  3. parseCharField → MakeString       [alloc: copies raw bytes]
  4. Function(1) → AllTrim RTL         [1 RTL Frame]
  5. strings.TrimSpace                  [alloc: new string]
  6. Return, continue

New opcode `PcOpFieldTrim <idx>` (0x47) fuses the two RTL calls into
a single opcode that:
  1. Calls FastFieldGetter directly (no Frame/Function dispatch).
  2. Walks the returned string with ASCII-space trim in place.
  3. Pushes `s[lo:hi]` — a sub-slice, no new allocation.
  4. Short-circuits back to the same string if no trim needed.

genpc recognizes the shape `AllTrim(FieldGet(<int-literal>))` in
emitCall and emits the fused opcode automatically — no SQL-side
API change. Matches the existing FieldGet peephole's shape.

Bench impact (50k rows, 3-run steady state, vs raw RDD baseline 6.2ms):

  String WHERE          before 7.9ms → after 6.2ms   1.00x (parity!)
  Numeric WHERE         6.9ms (unchanged)            1.11x
  No WHERE              9.1ms (unchanged)            1.47x

String WHERE is now at parity with the raw Harbour-style RDD scan.
Compared to session start (119ms), that's a 19x speedup.

Validation:
  - FiveSql2 43/43
  - Harbour compat 51/51
  - go test ./... ALL PASS

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

hbrt/pcinterp.go

@@ -174,6 +174,41 @@ func execPcodeBody(t *Thread, fn *PcodeFunc, mod *PcodeModule) {
 				t.Function(1)
 			}
 
+		// --- AllTrim(FieldGet(n)) fused peephole ---
+		case PcOpFieldTrim:
+			fIdx := int(binary.LittleEndian.Uint16(code[pc:]))
+			pc += 2
+			// Fast path: use direct field getter, trim inline.
+			var v Value
+			if fg := t.FastFieldGetter; fg != nil {
+				v = fg(fIdx)
+			} else {
+				// Fallback: resolve via FIELDGET RTL
+				t.PushSymbol(t.VM().FindSymbol("FIELDGET"))
+				t.PushNil()
+				t.PushLong(int64(fIdx))
+				t.Function(1)
+				v = t.Pop2()
+			}
+			if v.IsString() {
+				s := v.AsString()
+				// ASCII-space trim — DBF CHAR fields pad with 0x20 only
+				lo, hi := 0, len(s)
+				for lo < hi && s[lo] == ' ' {
+					lo++
+				}
+				for hi > lo && s[hi-1] == ' ' {
+					hi--
+				}
+				if lo == 0 && hi == len(s) {
+					t.PushValue(v)
+				} else {
+					t.PushString(s[lo:hi])
+				}
+			} else {
+				t.PushValue(v)
+			}
+
 		// --- Function calls ---
 		case PcOpPushSymbol:
 			slen := int(binary.LittleEndian.Uint16(code[pc:]))

hbrt/pcode.go

@@ -74,6 +74,12 @@ const (
 	// peephole optimization. Operand: uint16 1-based field position.
 	PcOpFieldGet byte = 0x46
 
+	// `AllTrim(FieldGet(n))` peephole — fetch the field, trim the
+	// result in place, push one string. Skips two Function dispatches
+	// (FieldGet + AllTrim) and one intermediate string allocation
+	// per invocation. Operand: uint16 1-based field position.
+	PcOpFieldTrim byte = 0x47
+
 	// Self / OOP
 	PcOpPushSelf      byte = 0x48
 	PcOpPushSelfField byte = 0x49 // + uint16 len + name