feat(FiveSql2): five_SQL block-callback integration — SQL beats raw PRG

Wires the new SqlEach RTL into FiveSql2's front-end so users write
the SQL they know and opt into streaming with a familiar Harbour
code block — no manual RTL plumbing.

API:

    /* Existing array form — unchanged, 43-test still green */
    aR := five_SQL( "SELECT name FROM t" )

    /* New block form — zero intermediate rows, 2x raw PRG */
    five_SQL( "SELECT id, name FROM t WHERE salary > 50000", NIL,
              {|nID, cName| Process(nID, cName)} )

Parameter order (cSQL, aParams, bBlock) keeps backward compatibility
with every existing call site. Passing NIL for aParams when only a
block is needed is standard Harbour idiom.

Routing:
  * TFiveSQL:Execute now takes an optional bBlock parameter and
    stores it on TSqlExecutor as ::bRowBlock.
  * TSqlExecutor:RunSelect's existing Go fast path (same guards as
    before: single table, no JOIN/GROUP/aggregate, plain column
    projections, WHERE compilable via SqlExprToPrg) branches on
    ::bRowBlock:
      - block present → SqlEach streams rows through the block
      - block absent  → SqlScan materializes into aRows (current path)
  * Post-processing (GROUP BY / ORDER BY / window / DISTINCT / LIMIT)
    runs on empty aRows when block mode fires — all are no-ops on
    empty input, so the sequence stays harmless.
  * RunSelect returns NIL (not {fields, rows}) when ::bRowBlock was
    used — signals "streaming semantics, all work done in the block".

Complex queries (JOIN, GROUP BY, subquery, window, ORDER BY not
matchable by an index, LIMIT/OFFSET, etc.) still fall back to the
array path even when a block is supplied — those genuinely require
materialization. Block mode is a fast-path opt-in, not a semantic
change.

End-to-end bench (50k rows, steady state — includes the user-side
loop/block for every row):

  Path                                   Time     Speedup vs raw
  ──────────────────────────────────────────────────────────────
  Raw PRG DO WHILE !Eof() + WHERE sum    7.6ms    1.00x
  five_SQL array + FOR                   7.7ms    ~same
  five_SQL + block (new)                 3.7ms    2.05x ← beats raw
  ──────────────────────────────────────────────────────────────
  Raw PRG no WHERE                       6.1ms    1.00x
  five_SQL + block, no WHERE             2.9ms    2.10x ← beats raw

SQL now pays for itself on end-to-end timing — not just competitive
with hand-rolled RDD loops, but faster than them. The layered cost
of FieldGet's Frame+RTL-dispatch that hand-written loops incur per
call is gone; the block-callback path captures *dbf.DBFArea directly
via FastFieldGetter and uses PcOpFieldGet to bypass dispatch in the
compiled WHERE predicate.

Validation:
  - FiveSql2 43/43 (array API unchanged)
  - Harbour compat 51/51
  - go test ./... ALL PASS

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

_FiveSql2/src/TFiveSQL.prg

@@ -22,7 +22,7 @@ CLASS TFiveSQL
    DATA aParams INIT {}
 
    METHOD New( aParams ) CONSTRUCTOR
-   METHOD Execute( cSQL )
+   METHOD Execute( cSQL, bBlock )
    METHOD ExecuteWith( cSQL, aParams )
 
 ENDCLASS
@@ -37,7 +37,7 @@ METHOD New( aParams ) CLASS TFiveSQL
 RETURN SELF
 
 
-METHOD Execute( cSQL ) CLASS TFiveSQL
+METHOD Execute( cSQL, bBlock ) CLASS TFiveSQL
 
    LOCAL aTokens, hQuery, aResult
 
@@ -54,6 +54,7 @@ METHOD Execute( cSQL ) CLASS TFiveSQL
    ENDIF
 
    ::oExec := TSqlExecutor():New( hQuery, ::aParams )
+   ::oExec:bRowBlock := bBlock
    aResult := ::oExec:Run()
 
 RETURN aResult