feat(pp,rtl): Tier 2 audit followups — JOIN hash + PP validation + C heuristic

Three medium-priority audit items in one commit, each independently
revertible.

  * **#18 JOIN hash-join fast path.** New std.ch shape:
        JOIN WITH <alias> TO <file> [FIELDS ...] ON <mfield> = <dfield>
    expands to a 6-arg __dbJoin call with the master/detail key
    field names. Runtime detects the extra args, builds an O(M)
    hash over the detail's key column, then probes per master row
    for O(N+M) total — vs the FOR form's O(N*M). For 1k×1k that's
    2k vs 1M operations; the gap widens with N. The original FOR
    form is unchanged and stays the fallback for arbitrary
    predicates. New helper dbHashKey type-tags the key string so
    `1` (numeric), `"1"` (string), and `.T.` (logical) don't
    collide in the bucket map.

  * **#38 PP rule result-marker validation.** ParseRule now walks
    the result template after parseMarkers and warns about every
    `<name>` (or `<(name)>` / `<.name.>` / `<{name}>` / `#<name>`
    / `<"name">`) that doesn't match a pattern marker. Warnings
    flow into pp.errors via handleDirective with the directive's
    filename:line, so a typo'd `<NaMe>` in an `#xcommand`
    case-sensitive rule fails the build with a clear diagnostic
    instead of silently producing broken expansions.

  * **#44 looksLikeInlineC heuristic strengthened.** Catches more
    of the common Harbour-PRG-with-C-inline-block shapes that
    used to fall through and produce cryptic Go-side errors:
    function-like #define, `extern "C"` linkage blocks, C return-
    type declarations (`int foo(`, `static char* bar(`), and the
    hb_ret*() helper family used by Harbour's C FFI return
    setters. Two small predicate helpers (allLetters,
    allIdentChars) keep the C-vs-Go disambiguation tight enough
    that legit Go code (`func name() int { ... }`) doesn't trip.

  * **#28 LIST/DISPLAY pagination** — explicitly deferred. Proper
    pagination requires interactive terminal handling (Inkey(0)
    for the keypress) which would hang in CI / batch mode. Will
    revisit when an interactive terminal layer needs it for
    other reasons.

Test fixtures: tests/std_ch/test_join_hash.prg verifies the new
ON-form path produces the same output as the FOR form would.
std.ch runner now stands at 16/16.

Other gates green:
  go test ./...      : PASS
  FiveSql2 SQL:1999  : 43/43
  Harbour compat     : 56/56
  std.ch suite       : 16/16
  FRB suite          : 7/7

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

tests/std_ch/run.sh

@@ -31,6 +31,7 @@ TESTS=(
    test_unsupported
    test_block_comma
    test_compound_lhs
+   test_join_hash
 )
 
 work="$(mktemp -d)"

tests/std_ch/test_join_hash.prg

@@ -0,0 +1,50 @@
+/* JOIN ... ON hash-join fast path. */
+
+PROCEDURE Main()
+   LOCAL n
+
+   FErase("cust.dbf"); FErase("ord.dbf"); FErase("out.dbf")
+
+   /* Customers (master) */
+   dbCreate("cust.dbf", { {"CID","N",4,0}, {"CNAME","C",12,0} })
+   USE cust.dbf NEW EXCLUSIVE ALIAS cu
+   dbAppend(); FieldPut(1, 1); FieldPut(2, "Alice")
+   dbAppend(); FieldPut(1, 2); FieldPut(2, "Bob")
+   dbAppend(); FieldPut(1, 3); FieldPut(2, "Carol")
+   dbCommit()
+   dbCloseArea()
+
+   /* Orders (detail) */
+   dbCreate("ord.dbf", { {"OID","N",4,0}, {"CID","N",4,0}, {"AMT","N",8,2} })
+   USE ord.dbf NEW EXCLUSIVE ALIAS od
+   dbAppend(); FieldPut(1, 100); FieldPut(2, 1); FieldPut(3, 50)
+   dbAppend(); FieldPut(1, 101); FieldPut(2, 1); FieldPut(3, 30)
+   dbAppend(); FieldPut(1, 102); FieldPut(2, 2); FieldPut(3, 200)
+   dbAppend(); FieldPut(1, 103); FieldPut(2, 4); FieldPut(3, 99) /* no-match */
+   dbCommit()
+   dbCloseArea()
+
+   USE cust.dbf NEW EXCLUSIVE ALIAS cu
+   USE ord.dbf  NEW EXCLUSIVE ALIAS od
+   SELECT cu
+
+   /* Hash-join form */
+   JOIN WITH od TO out.dbf FIELDS cid, cname, oid, amt ON cid = cid
+
+   SELECT cu ; dbCloseArea()
+   SELECT od ; dbCloseArea()
+
+   USE out.dbf NEW EXCLUSIVE
+   COUNT TO n
+   ? "Hash-joined rows =", n, "(expect 3)"
+   dbGoTop()
+   DO WHILE !Eof()
+      ? "  ", FieldGet(1), AllTrim(FieldGet(2)), FieldGet(3), FieldGet(4)
+      dbSkip()
+   ENDDO
+   ? "(expect three rows: Alice/100/50, Alice/101/30, Bob/102/200)"
+   dbCloseArea()
+
+   FErase("cust.dbf"); FErase("ord.dbf"); FErase("out.dbf")
+   ? "DONE"
+   RETURN