docs+tools: workflow flowchart harness (P4) + drivers Tier-1/Tier-2 split

- tools/flowgen.py: generates docs/workflows/<name>.md — one outcome-first mermaid
  flowchart per workflow the SDK tests (hpmcu watchdog, modbus read, cru ladder,
  rga offload, relay drive, freshness contract), each stamped with its benchmark
  ns/op or 100% MC/DC result. Deterministic; reads the cargo-bench trend json.
- drivers/README.md: honest Tier-1 (relays + freshness, 100% MC/DC here now) vs
  Tier-2 (modbus/rga — serious testing + fault-injection + benchmarks via sim
  models; driver sources migrate in with the flare-edge unification) per ADR-0002/0005.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017wB8KB3MMQztRDXCMCkPrf

docs/workflows/modbus-read-holding.md

@@ -0,0 +1,18 @@
+# Modbus RTU: read-holding-registers round trip
+
+> **Outcome tested:** A well-formed request yields the right registers; a bad one a defined fault.
+
+**Benchmark** (`modbus_read_holding`): 88.0 ns/op
+
+```mermaid
+flowchart TD
+  A[frame in] --> B{addr == mine\nor broadcast?}
+  B -- no --> Z[ignore]
+  B -- yes --> C{CRC ok?}
+  C -- no --> Z
+  C -- yes --> D{function code}
+  D -- 0x03 read-holding --> E{range in bounds?}
+  E -- no --> X[exception 0x02]
+  E -- yes --> R[registers response + CRC]
+  D -- unsupported --> X2[exception 0x01]
+```