fix(toolservers): replace all banned V-lang adapters with Zig — dap, lang, bsp

V-lang banned 2026-04-10. Three more adapter replacements:
  dap-mcp:  dap_adapter.v  → dap_adapter.zig  (REST :9019, gRPC :9020, GraphQL :9021)
  lang-mcp: lang_adapter.v → lang_adapter.zig  (REST :9022, gRPC :9023, GraphQL :9024)
  bsp-mcp:  bsp_adapter.v  → bsp_adapter.zig  (REST :9025, gRPC :9026, GraphQL :9027)

Each adapter imports its sibling ffi module directly (no shared-library linking).
lang-mcp/ffi/build.zig added (was missing). All three adapters have build.zig.

lang_adapter.zig adds string-buffer FFI handling for lang_typecheck + lang_eval
(src/out via ptr+len pairs) with jsonQuote escaping for response embedding.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: hyperpolymath