docs: mesh multiplayer protocol with dot diagrams

Adds docs/mesh.md and five graphviz diagrams covering the federation
layer: identity stack, epoch state machine, per-member secret envelope,
gossip wire contract, and operator decision tree. Pins the protocol
contract for the upcoming HTTP wire (mesh sync, mesh serve).

Makefile gets a 'docs' target with pattern rule so PNG renders are
incremental from .dot sources.

Makefile

@@ -30,7 +30,7 @@ SEARCH_Q     ?= computer
         ingest-xml-attached ingest-abstract \
         ingest-grok ingest-grok-media \
         ingest-self ingest-git ingest-hg \
-        verify search stats test clean clean-db clean-data help
+        verify search stats test docs clean clean-db clean-data help
 
 all: bootstrap fetch-cur ingest-cur verify stats ## bootstrap → fetch cur → ingest cur → verify → stats
 
@@ -282,6 +282,14 @@ stats: bootstrap ## counts: documents, chunks, edges, audit chain
 test: bootstrap ## run pytest suite
 	$(VENV)/bin/pytest -q
 
+DOT_SRCS := $(wildcard docs/diagrams/*.dot)
+DOT_PNGS := $(DOT_SRCS:.dot=.png)
+
+docs/diagrams/%.png: docs/diagrams/%.dot
+	dot -Tpng $< -o $@
+
+docs: $(DOT_PNGS) ## render docs/diagrams/*.dot -> .png via graphviz
+
 # Reproducible micro-benchmark over a fixed slice of cur. Lets you compare
 # ETL throughput across configs and catches regressions on optimization
 # work. Override BENCH_DOCS=N (default 5000).