fix sandbox app script loading and timer execution

- Add relative path resolution for filesystem-loaded documents in kernel.cpp
  - SetDocumentBasePath() tracks document directory for relative resource resolution
  - ResolvePath() resolves relative paths like "app.lua" against document base path
- Fix RmlUi context name lookup in sandbox test app (use "default" not "main")
- Add debug logging to timer_manager.cpp to trace timer creation and execution

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

test-apps/com.mosis.sandbox-test/app.lua

@@ -16,11 +16,12 @@ local function getDocument()
         currentDocument = document
         return document
     end
-    -- Try to get from RmlUi context
-    if rmlui and rmlui.contexts and rmlui.contexts.main then
-        local ctx = rmlui.contexts.main
-        -- documents is a proxy, iterate to get first doc
-        if ctx.documents then
+    -- Try to get from RmlUi context (our context is named "default")
+    if rmlui and rmlui.contexts then
+        -- Try both "default" and "main" context names
+        local ctx = rmlui.contexts["default"] or rmlui.contexts.main
+        if ctx and ctx.documents then
+            -- documents is a proxy, iterate to get first doc
             for i, doc in ipairs(ctx.documents) do
                 if doc then
                     currentDocument = doc
@@ -155,7 +156,7 @@ function testCrypto()
 
     -- Test SHA256
     if success then
-        local hash = crypto.sha256("hello world")
+        local hash = crypto.hash("sha256", "hello world")
         if hash then
             log("SHA256: " .. hash:sub(1, 32) .. "...")
         else