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/hpmcu-watchdog.md

@@ -0,0 +1,17 @@
+# HPMCU watchdog: arm → beat → fire
+
+> **Outcome tested:** A hung A7/flared ends in a counted reset, not a dark panel.
+
+**Benchmark** (`hpmcu_tick`): 1.8 ns/op
+
+```mermaid
+flowchart TD
+  A[flared loads SCR1 fw, releases core] --> B[MCU tick]
+  B --> C{mailbox magic == DISARM?}
+  C -- yes --> D[disarmed: never fire]
+  C -- no --> E{magic == ARMED?}
+  E -- no --> B
+  E -- yes --> F{beat counter advanced\nwithin deadline?}
+  F -- yes --> B
+  F -- no --> G[fire CRU global reset]
+```