feat(v2): M4 Memory Manager + Prefetch Engine, real V1 PSRAM backend

Implements M4: memory_manager.v (arbitration/buffering/forwarding/
latency hiding/double buffering, §12) + prefetch_engine.v
(double-buffered tile fetch, §13), sitting on the REAL, UNMODIFIED V1
PSRAM backend chain (int8_memory_access.v -> memory_interface.v ->
psram_controller.v, per §15's explicit mandate not to touch the
controller).

Verified fully end-to-end with Verilator: real neural_processor (M1)
fed entirely by memory_manager, computing against PSRAM-resident X/W
tiles (double-buffered prefetch across up to 5 tiles) and writing its
result back to PSRAM -- checked via an independent PSRAM read-back,
with poison bytes around the operand regions to catch addressing
errors. 3/3 jobs pass (1/3/5-tile configurations).

Three real RTL bugs found and fixed during integration (full
diagnostic trail in errors.log ERR-0006): prefetch_engine had no
single-in-flight-request discipline, letting a queued request corrupt
the bank bookkeeping of a fetch already running; the fix's own
!pf_busy guard had a one-cycle blind spot (pf_busy lags pf_start by a
clock) that needed an explicit !pf_start term; and a state-based mux
for the shared backend port was off by one cycle, silently dropping
the PSRAM result write entirely.

Real synthesis: 0 CHECK problems, 851 LUT4/789 FF/108 CCU2C/0 DSP
(expected, no multiplication in this module). Real place&route (via a
synthesis-only timing harness, needed for the same TRELLIS_IO pin-
budget reason as M2's array): Fmax 165.86 MHz, PASS at 80MHz.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013xXuuRUWZScuo1DeYJxs3v

hardware/v2/logs/synthesis.log

@@ -36,3 +36,7 @@ result_buffer     D=4096: LUT=37 FF=30  DP16KD=2  (D=256: LUT=21 FF=26 DP16KD=1)
 CHECK: 0 problems, all 6 configs correctly infer DP16KD (no LUT-RAM
   fallback). weight_buffer's DP16KD count does NOT drop with depth
   (width-bound, not depth-bound -- see decisions.log / benchmark.log).
+
+[2026-09-05] EXP-0005 -- memory_manager + prefetch_engine (standalone)
+LUT4=851  TRELLIS_FF=789  CCU2C=108  MULT18X18D=0 (expected, no
+  multiplication in this module). CHECK: 0 problems.