chore: initial commit for v0.0.1

DChain single-node blockchain + React Native messenger client.

Core:
- PBFT consensus with multi-sig validator admission + equivocation slashing
- BadgerDB + schema migration scaffold (CurrentSchemaVersion=0)
- libp2p gossipsub (tx/v1, blocks/v1, relay/v1, version/v1)
- Native Go contracts (username_registry) alongside WASM (wazero)
- WebSocket gateway with topic-based fanout + Ed25519-nonce auth
- Relay mailbox with NaCl envelope encryption (X25519 + Ed25519)
- Prometheus /metrics, per-IP rate limit, body-size cap

Deployment:
- Single-node compose (deploy/single/) with Caddy TLS + optional Prometheus
- 3-node dev compose (docker-compose.yml) with mocked internet topology
- 3-validator prod compose (deploy/prod/) for federation
- Auto-update from Gitea via /api/update-check + systemd timer
- Build-time version injection (ldflags → node --version)
- UI / Swagger toggle flags (DCHAIN_DISABLE_UI, DCHAIN_DISABLE_SWAGGER)

Client (client-app/):
- Expo / React Native / NativeWind
- E2E NaCl encryption, typing indicator, contact requests
- Auto-discovery of canonical contracts, chain_id aware, WS reconnect on node switch

Documentation:
- README.md, CHANGELOG.md, CONTEXT.md
- deploy/single/README.md with 6 operator scenarios
- deploy/UPDATE_STRATEGY.md with 4-layer forward-compat design
- docs/contracts/*.md per contract

node/api_well_known_version.go

@@ -0,0 +1,93 @@
+// Package node — /api/well-known-version endpoint.
+//
+// Clients hit this to feature-detect the node they're talking to, without
+// hardcoding "node >= 0.5 supports channels" into every screen. The response
+// lists three coordinates a client cares about:
+//
+//   - node_version     — human-readable build tag (ldflags-injectable)
+//   - protocol_version — integer bumped only on wire-protocol breaking changes
+//   - features         — stable string tags for "this binary implements X"
+//
+// Feature tags are ADDITIVE — once a tag ships in a release, it keeps being
+// returned forever (even if the implementation moves around internally). The
+// client uses them as "is this feature here or not?", not "what version is
+// this feature at?". Versioning a feature is done by shipping a new tag
+// (e.g. "channels_v2" alongside "channels_v1" for a deprecation window).
+//
+// Response shape:
+//
+//	{
+//	  "node_version":     "0.5.0-dev",
+//	  "protocol_version": 1,
+//	  "features": [
+//	    "channels_v1",
+//	    "fan_out",
+//	    "native_username_registry",
+//	    "ws_submit_tx",
+//	    "access_token"
+//	  ],
+//	  "chain_id": "dchain-ddb9a7e37fc8"
+//	}
+package node
+
+import (
+	"fmt"
+	"net/http"
+	"sort"
+
+	"go-blockchain/node/version"
+)
+
+// ProtocolVersion is bumped only when the wire-protocol changes in a way
+// that a client compiled against version N cannot talk to a node at
+// version N+1 (or vice versa) without updating. Adding new optional fields,
+// new EventTypes, new WS ops, new HTTP endpoints — none of those bump this.
+//
+// Bumping this means a coordinated release: every client and every node
+// operator must update before the old version stops working.
+const ProtocolVersion = 1
+
+// nodeFeatures is the baked-in list of feature tags this binary implements.
+// Append-only. When you add a new tag, add it here AND document what it means
+// so clients can feature-detect reliably.
+//
+// Naming convention: snake_case, versioned suffix for anything that might get
+// a breaking successor (e.g. `channels_v1`, not `channels`).
+var nodeFeatures = []string{
+	"access_token",              // DCHAIN_API_TOKEN gating on writes (+ optional reads)
+	"channels_v1",               // /api/channels/:id + /members with X25519 enrichment
+	"chain_id",                  // /api/network-info returns chain_id
+	"contract_logs",             // /api/contract/:id/logs endpoint
+	"fan_out",                   // client-side per-recipient envelope sealing
+	"identity_registry",         // /api/identity/:pub returns X25519 pub + relay hints
+	"native_username_registry",  // native:username_registry contract
+	"onboarding_api",            // /api/network-info for joiner bootstrap
+	"payment_channels",          // off-chain payment channel open/close
+	"relay_mailbox",             // /relay/send + /relay/inbox
+	"ws_submit_tx",              // WebSocket submit_tx op
+}
+
+func registerWellKnownVersionAPI(mux *http.ServeMux, q ExplorerQuery) {
+	mux.HandleFunc("/api/well-known-version", func(w http.ResponseWriter, r *http.Request) {
+		if r.Method != http.MethodGet {
+			jsonErr(w, fmt.Errorf("method not allowed"), 405)
+			return
+		}
+		// Return a copy so callers can't mutate the shared slice.
+		feats := make([]string, len(nodeFeatures))
+		copy(feats, nodeFeatures)
+		sort.Strings(feats)
+
+		resp := map[string]any{
+			"node_version":     version.Tag,
+			"build":            version.Info(),
+			"protocol_version": ProtocolVersion,
+			"features":         feats,
+		}
+		// Include chain_id if the node exposes it (same helper as network-info).
+		if q.ChainID != nil {
+			resp["chain_id"] = q.ChainID()
+		}
+		jsonOK(w, resp)
+	})
+}