ADR-0033 D6: clarify what multi-flow merging actually models

Earlier the future-work list mentioned "multi-flow fair sharing on a
single shared link" which was confusing — each wire has a single
source, so this isn't a real gap. The actual modeling story:

- Multi-stream merging at routers IS handled via per-in_port fan_in +
  shared inbox + FIFO worker forwarding. Flits from different
  upstream streams interleave at flit granularity naturally.
- What's NOT modeled: cycle-accurate arbitration policies (priority,
  iSLIP), address-based PC selection at HBM CTRL (round-robin is
  address-blind, so size-aligned concurrent transactions hit full
  PC contention even when real-HW address striping would diverge),
  sub-flit (32B) granularity, finite buffer backpressure, and bank
  conflict modeling.

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

docs/adr/ADR-0033-latency-model-assumptions.md

@@ -102,18 +102,39 @@ accurate for **relative comparisons** within the modeled regime.
 
 ### D6. Future work
 
-- [ ] Bank-level conflict modeling (opt-in via `track_banks: true`).
-- [ ] HBM scheduler with write buffer + watermark drain (Tier 2 from the
-  design discussion).
-- [ ] Fluid wire model for multi-flow fairness on a single shared link
-  (currently FIFO serial).
-- [ ] Sub-flit (32B) granularity for cycle-accurate wire arbitration.
-- [ ] Backpressure modeling for finite component buffers.
-- [ ] Op_log integration with chunk-streaming (currently op_log fires on
-  PE-internal command messages — DmaReadCmd, DmaWriteCmd, GemmCmd,
-  MathCmd — which are not chunkified; integration would require
-  flit-aware components to also emit op_log start/end hooks per
-  transaction).
+Note: multi-stream merging at routers IS modeled correctly — each
+in_port has its own fan_in process, all push to a shared inbox, and
+the router worker forwards in inbox FIFO order. Flits from different
+upstream streams naturally interleave at flit granularity. The items
+below are different concerns.
+
+- [ ] **Cycle-accurate router arbitration policies** (RR with
+  priorities, age, iSLIP). Currently the inbox FIFO order is used as
+  a proxy for fair RR — works when flit arrival times differ slightly
+  between streams, but doesn't reflect intentional priority/QoS.
+- [ ] **Sub-flit (32B) granularity** for finer wire arbitration
+  cycles. Our `flit_bytes` equals burst (256B); real HW arbitrates
+  per 32B flit. Effect is small for most workloads (sub-flit timing
+  noise).
+- [ ] **Address-based PC selection at HBM CTRL** (replace the
+  address-blind global round-robin). When two transactions of size
+  `num_pcs × burst_bytes` (e.g., 2KB at 8 PCs × 256B) arrive
+  concurrently, both claim PCs 0..7 via global RR, producing full
+  per-PC contention. Real HW uses address bits to select PCs, so
+  different-address transactions hit different PC patterns. Address
+  modeling would let the simulator reflect cache-line/page-aware
+  layouts.
+- [ ] **Bank-level conflict modeling** within a PC (opt-in via
+  `track_banks: true`). Currently we assume no same-bank reuse.
+- [ ] **HBM scheduler** with write buffer + watermark drain (Tier 2
+  from the design discussion). Default `switch_penalty_ns=0` is the
+  ideal-amortization stand-in.
+- [ ] **Backpressure** modeling for finite component buffers.
+- [ ] **Op_log integration with chunk-streaming**: currently op_log
+  fires on PE-internal command messages (DmaReadCmd, DmaWriteCmd,
+  GemmCmd, MathCmd) which are not chunkified. Integration would
+  require flit-aware components to also emit op_log start/end hooks
+  per transaction (start on first flit, end on is_last).
 
 ## Consequences