PINK DITAv2: fix 4 Critical/High flaws (I1, G2, G3, I13, I18)

- I1 (Critical/Rust): apply_fill accumulated partial fills instead of
  overwriting. WS events carry lastFilledQty (incremental); previous code
  set slot.size = fill_size each time. Now accumulates via prev_filled.
  initial_size set from intended_size on first fill, not from fill amount.

- G2 (Critical/Rust): into_c_string unwrap() panicked on any NUL byte in
  serialized JSON. Now sanitizes NUL bytes before CString construction;
  never panics.

- G3 (Critical/Rust): EXIT intent transition hardcoded prev_state=
  POSITION_OPEN. Captured actual fsm_state before mutation so audit trail
  is accurate when EXIT is received from non-standard states.

- I13 (High/Rust): stray venue event could reactivate a closed slot.
  Added explicit slot.closed guard in on_venue_event — returns
  TERMINAL_STATE with accepted=false before any FSM mutation.

- I18 (High/Python): sys.path.insert(0, ...) in real_zinc_plane.py and
  real_control_plane.py gave Zinc adapter directory highest import
  priority. Changed to sys.path.append() so existing path entries take
  precedence.

35/35 offline tests pass.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

prod/clean_arch/dita_v2/real_zinc_plane.py

@@ -21,7 +21,7 @@ from .control import KernelControlSnapshot
 
 _ZINC_ADAPTER_PATH = Path(__file__).resolve().parents[3] / "zinc" / "adapters" / "python"
 if _ZINC_ADAPTER_PATH.exists() and str(_ZINC_ADAPTER_PATH) not in sys.path:
-    sys.path.insert(0, str(_ZINC_ADAPTER_PATH))
+    sys.path.append(str(_ZINC_ADAPTER_PATH))
 
 try:  # pragma: no cover - exercised in integration tests
     from zinc import SharedRegion