docs: real P&R directive tuning closes most of remaining N=16 timing gap (EXP-0094 cont.)

Same RTL, real Vivado strategy directives (Explore/ExtraNetDelay_high/
AggressiveExplore, zero RTL risk): WNS -0.646ns -> -0.338ns, TNS
-97.5ns -> -5.3ns, failing endpoints 771 -> 60. Remaining bottleneck
confirmed intrinsic (84% logic delay, DSP48E1->CARRY4 path inside
neural_processor_packed.v, recurring per-PE) -- unlikely to shrink
further via P&R strategy alone.

Cumulative: two safe fixes (hierarchical arbiter + directive tuning)
close ~89% of the original TNS gap and ~63% of WNS, neither touching
neural_processor_packed.v. Deliberate stop: closing the rest needs
that shared, load-bearing module touched -- a larger, more careful
step (verify against both N=2 and N=16) documented as next_action,
not attempted without further direction.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MUG92aM9m68TRc4rG55BcC

docs/ARCHITECTURE_ANALYSIS.md

@@ -659,7 +659,7 @@ specifically to document where/how it breaks rather than to succeed):
 
 ---
 
-### 5.6 [Functionally DONE, TIMING SUBSTANTIALLY IMPROVED BUT NOT YET CLOSED — EXP-0089/0090/0091/0092/0093/0094] Hybrid systolic scaling: 4 groups × 4-PE weight-stationary chains
+### 5.6 [Functionally DONE, WNS -0.913ns→-0.338ns via 2 safe fixes, small intrinsic gap remains — EXP-0089…0094] Hybrid systolic scaling: 4 groups × 4-PE weight-stationary chains
 
 Captured from a 2026-09-20 brainstorming session as a purely exploratory
 idea; the same day, per the user's own explicit reprioritization, the real
@@ -777,17 +777,39 @@ NOT fully closed timing result (EXP-0094)** — honestly not yet ready
 for real hardware at the target clock. N=2 (EXP-0088) remains the
 real, trustworthy, deployable signoff.
 
+- **Real P&R strategy tuning (EXP-0094, continued)**: same RTL,
+  `opt_design -directive Explore` + `place_design -directive
+  ExtraNetDelay_high` + a new `phys_opt_design -directive
+  AggressiveExplore` step + `route_design -directive
+  AggressiveExplore` — zero RTL risk. **Real result: WNS improved
+  further to −0.338ns, TNS to −5.299ns, failing endpoints to 60** —
+  another real, substantial, measured improvement (recovered roughly
+  half of the remaining gap). Still not fully met. The new worst path
+  is the SAME structural class (a different PE instance, confirming a
+  recurring per-PE issue) but now measured at 84% logic delay (up from
+  79%) — real evidence the recoverable route-delay slack is largely
+  exhausted; what remains is dominated by the FPGA's own intrinsic
+  DSP48E1→CARRY4 interconnect delay, unlikely to shrink further via
+  more P&R strategy tuning alone.
+
+**Real, cumulative progress**: two safe, real improvements (the
+hierarchical arbiter + P&R directive tuning), neither touching
+`neural_processor_packed.v`, together close ~89% of the original TNS
+gap (−690ns → −5.3ns) and ~63% of the original WNS gap (−0.913ns →
+−0.338ns). **Deliberate stop here**: the remaining gap needs the
+shared, load-bearing `neural_processor_packed.v` MAC datapath itself
+touched to close via RTL — a meaningfully larger, more careful step
+(must be verified against both N=2's own signoff and N=16) than
+anything else this session, better started with explicit direction
+than pushed further autonomously.
+
 **Not yet done, real and disclosed, real options for closing the
-remaining gap** (need a real decision on direction before committing
-more RTL effort — `neural_processor_packed.v` is the compute core
-shared by every real PE in this project): (1) real pipelining inside
+remaining gap**: (1) real, careful pipelining inside
 `neural_processor_packed.v`'s own MAC datapath at the specific
 `GEN_MAC_PACKED`/`prodb1_reg` boundary (fork-before-promote, N=2's own
-signoff must stay protected). (2) a real, measured lower target clock
-for the N=16 variant specifically (unquantified throughput trade-off
-against N=2). (3) real Vivado placement/timing directives (e.g. a
-`PBLOCK` per group to reduce congestion) as a lower-RTL-risk first
-attempt. See EXP-0094's own `next_action`.
+signoff must stay protected, verify both). (2) a real, measured lower
+target clock for the N=16 variant specifically (unquantified
+throughput trade-off against N=2). See EXP-0094's own `next_action`.
 
 **The problem it targets**: plain N=16 independent cores (§5.5's own
 "documentary, expected to break" framing) means 16 independent DDR3