docs: split-mode setup guide, architecture diagrams, cert script, build

README:
- New 'Split-mode setup' section: motivation, firewall rules, step-by-step
  instructions for both VPN tunnel and TLS distribution options
- Split-mode bullet added to Features list
- Notes on DPI-resistant tunnels (Shadowsocks, VLESS/XRay, Hysteria2) for
  Russian deployment; standard VPN protocols (WireGuard, OpenVPN) may be blocked
- Install instructions updated to use `make init-config` (copies templates,
  auto-detects public IP) instead of manual cp; ROLE= documented throughout
- Split-mode Step 4 uses `make ROLE=back/front` so template-change detection
  works correctly after `git pull`

Makefile:
- ROLE ?= both variable selects config templates (both/front/back)
- Config prereq rules use $(SYS_CONFIG_SRC) / $(VM_ARGS_SRC) based on ROLE
- New `init-config` target: force-copies templates, auto-detects public IP,
  prints edit reminder; replaces manual cp in install workflow

scripts/gen_dist_certs.sh:
- Two-step workflow: `init <dir>` on back server (CA + back cert),
  `add-node <dir> <name>` per front server (cert signed by existing CA)
- Generates per-node ssl_dist.<name>.conf with paths substituted (no
  NODE_NAME placeholder to edit manually)
- ssl_dist.<name>.conf is now used directly (no rename to ssl_dist.conf);
  vm.args examples and README updated to match

config/vm.args.{front,back}.example:
- -ssl_dist_optfile points to role-specific filename (ssl_dist.front.conf /
  ssl_dist.back.conf) so cert files can be copied as-is without renaming

AGENTS.md:
- Role-overview Mermaid flowchart showing front/back/both process split
- Data-plane section replaced with links to doc/ (no duplication)
- Supervision tree, key interactions, split-mode config keys updated

doc/handler-downstream-flow.md, doc/migration-flow.md:
- Mermaid box grouping to visually separate FRONT and BACK node participants
- erpc:call reference corrected (was rpc:call)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

config/vm.args.front.example

@@ -28,17 +28,18 @@
 ##   - If front is NOT in a censored region, WireGuard/IPsec work fine too.
 ##
 ## Option B: TLS distribution (no tunnel required)
-##   - On back server: scripts/gen_dist_certs.sh init /etc/mtproto-proxy/dist
-##   - Run per front:  scripts/gen_dist_certs.sh add-node /etc/mtproto-proxy/dist front
+##   - On back server: `scripts/gen_dist_certs.sh init /etc/mtproto-proxy/dist`
+##   - Run per front:  `scripts/gen_dist_certs.sh add-node /etc/mtproto-proxy/dist front`
 ##   - Place ca.pem, front.pem, front.key in /etc/mtproto-proxy/dist/ here.
-##   - Deploy ssl_dist.front.conf as /etc/mtproto-proxy/dist/ssl_dist.conf
+##   - Place ssl_dist.front.conf in /etc/mtproto-proxy/dist/ on the front server.
+##   - Uncomment the lines below:
 ##
-## -proto_dist inet_tls
-## -ssl_dist_optfile /etc/mtproto-proxy/dist/ssl_dist.conf
+# -proto_dist inet_tls
+# -ssl_dist_optfile /etc/mtproto-proxy/dist/ssl_dist.front.conf
 ##
 ## Firewall: allow TCP on the distribution port (9199 below) only between
 ## the front and back servers, never to the public internet.
 ##
-## -kernel inet_dist_listen_min 9199
-## -kernel inet_dist_listen_max 9199
+# -kernel inet_dist_listen_min 9199
+# -kernel inet_dist_listen_max 9199
 ## -----------------------------------------------------------------------