test(mods): document and cover streamed import access

tests/engine/stream_md5_test.lua

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+-- Incremental MD5 vectors used by large required-import streaming.
+package.path = "./?.lua;./?/init.lua;" .. package.path
+
+-- Plain Lua runners may expose bit32 while production LuaJIT exposes bit.
+if not rawget(_G, "bit") and not rawget(_G, "bit32") then
+  local ok, bit32 = pcall(require, "bit32")
+  if ok then _G.bit32 = bit32 end
+end
+
+local T = require("tests.modkit")
+local MD5 = require("src.mods.StreamMD5")
+
+local vectors = {
+  { "", "d41d8cd98f00b204e9800998ecf8427e" },
+  { "a", "0cc175b9c0f1b6a831c399e269772661" },
+  { "abc", "900150983cd24fb0d6963f7d28e17f72" },
+  { "message digest", "f96b697d7cb7938d525a2f31aaf161d0" },
+  { "abcdefghijklmnopqrstuvwxyz", "c3fcd3d76192e4007dfb496cca67e13b" },
+}
+
+for _, row in ipairs(vectors) do
+  local ctx = MD5.new()
+  for i = 1, #row[1], 3 do ctx:update(row[1]:sub(i, i + 2)) end
+  T.eq(ctx:final(), row[2], "incremental MD5 vector: " .. row[1])
+end
+
+-- Cross a large number of block boundaries without building one giant string.
+local million = MD5.new()
+for _ = 1, 1000 do million:update(string.rep("a", 1000)) end
+T.eq(million:final(), "7707d6ae4e027c70eea2a935c2296f21",
+  "RFC 1321 million-a vector")
+
+T.finish("stream_md5")