feat: DPDK Market Data Feed Handler — production product app

Add complete market data gateway product:
- app/market_feed.c: Zero-copy DPDK feed handler with latency histogram
  (min/avg/P50/P95/P99/max), per-symbol msg rate, periodic stats, signal
  handling, JSON parsing without library dependency.
- demo/market_data_server.py: Zero-dependency simulated market data
  server (8 symbols, random walk pricing, subscribe/unsubscribe).
- app/README.md: Full product documentation with architecture diagram.
- Makefile: Add 'app' target for product build (build/market_feed).
- src/main.c: Add -M market data streaming mode.

Product is ready for: financial market data gateways, low-latency
quote feeds, exchange connectivity testing.

Co-Authored-By: Claude <noreply@anthropic.com>

Makefile

@@ -34,7 +34,17 @@ LDFLAGS  = $(DPDK_LIBS) -lpthread -ldl -lnuma
 LDFLAGS += -Wl,--allow-multiple-definition
 LDFLAGS += -Wl,-rpath,$(DPDK_BUILD)/lib
 
-.PHONY: all clean clean-all perf
+# Market Data Feed Handler (product app)
+FEED_SRCS := app/market_feed.c
+FEED_SRCS += src/tcp_core.c
+FEED_SRCS += src/tcp_input.c
+FEED_SRCS += src/tcp_output.c
+FEED_SRCS += src/tcp_timer.c
+FEED_SRCS += src/tcp_socket.c
+FEED_SRCS += src/ws_client.c
+FEED_OBJS := $(FEED_SRCS:%.c=$(BUILD_DIR)/%.o)
+
+.PHONY: all clean clean-all perf app
 
 all: $(BUILD_DIR)/$(APP)
 
@@ -71,3 +81,17 @@ clean:
 
 clean-all: clean
 	rm -f $(APP)
+
+# ── Market Data Feed Handler (production app) ─────────────────
+
+app: $(BUILD_DIR)/market_feed
+
+$(BUILD_DIR)/market_feed: $(FEED_OBJS)
+	@mkdir -p $(@D)
+	PKG_CONFIG_PATH="$(PKG_CONFIG_PATH)" gcc $(FEED_OBJS) $(LDFLAGS) -o $@
+	@echo "  LINK  $@"
+
+$(BUILD_DIR)/app/%.o: app/%.c
+	@mkdir -p $(@D)
+	PKG_CONFIG_PATH="$(PKG_CONFIG_PATH)" gcc $(CFLAGS) -Isrc/ -MMD -c $< -o $@
+	@echo "  CC    $<"