feat(rdd): real POSIX file/record locking + gap analysis doc

Replaces the FLOCK/DBRLOCK/DBRUNLOCK no-op stubs with actual
fcntl(F_SETLK) byte-range advisory locks, matching Harbour's
hb_fsLockLarge implementation.

Before: rtlDbRLock always returned .T. regardless of contention.
        Multi-process writers could silently corrupt records.

After:  Non-blocking POSIX byte-range locks per file descriptor.
        Cross-process exclusion verified by a subprocess-spawning
        Go test that witnesses BUSY vs OK transitions.

New files:
  hbrdd/dbf/locks_posix.go    fcntl F_WRLCK/F_UNLCK wrappers
  hbrdd/dbf/locks_windows.go  stub (TODO: LockFileEx)
  hbrdd/dbf/lock_multi_test.go   cross-process verification
  docs/gap-analysis.md        honest Harbour parity assessment

Modified:
  hbrdd/dbf/dbf.go
    - DBFArea gains fileLocked bool + lockedRecs map
    - Close() calls releaseAllLocks() before dropping the fd
  hbrtl/database.go
    - rtlDbRLock / rtlDbRUnlock now delegate to DBFArea.LockRecord /
      UnlockRecord instead of returning fixed .T./NIL
    - New rtlFLock / rtlDbUnlock for FLOCK() / DBUNLOCK()
  hbrtl/register.go
    - FLOCK and DBUNLOCK symbols registered (were missing entirely)
  compiler/analyzer/analyzer.go
    - FLOCK / DBUNLOCK added to RTL known-function set

Lock region layout (non-overlapping on purpose):
  FLOCK region       [0, HeaderLen+1)
  Record N region    [RecordOffset(N), RecordLen)

So a workarea can hold FLOCK and multiple DBRLOCK simultaneously
on the same fd without conflict.

Design rationale (captured in locks_posix.go header):
  * POSIX fcntl, not flock(2) — byte-range + NFS-safe
  * Non-blocking F_SETLK — matches Clipper FLOCK() → .F. semantics
  * Released explicitly on Close to avoid workarea-sharing races
  * Windows falls back to no-op (TODO: LockFileEx)

Verification:
  go test ./hbrdd/dbf/ -run TestFLockBlocksAcrossProcesses  PASS
  go test ./hbrdd/dbf/ -run TestRLockBlocksAcrossProcesses  PASS
  go test ./...                                             ALL PASS
  FiveSql2 43/43                                            100%
  compat_harbour 51/51                                      100%

The gap-analysis doc (docs/gap-analysis.md) is a running inventory
of what works vs what's still missing vs Harbour 3.2, written for
users evaluating Five for production — not a sales pitch.

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

compiler/analyzer/analyzer.go

@@ -488,6 +488,7 @@ var rtlFunctions = map[string]bool{
 	"DBSEEK": true, "DBSELECTAREA": true, "DBPACK": true, "DBZAP": true,
 	"DBCREATE": true, "DBINFO": true, "DBORDERINFO": true, "DBSETINDEX": true,
 	"RECALL": true, "PACK": true, "ZAP": true,
+	"FLOCK": true, "DBUNLOCK": true,
 	"__DBPACK": true, "__DBZAP": true,
 	// Locate/Filter
 	"DBLOCATE": true, "__DBLOCATE": true, "__DBCONTINUE": true,