Update docs for DC-1 testing, fix toolbar clipping

- Updated DESIGN.md: rendering strategy (no backing bitmap, screen-space
  grid, viewport clamping), added line snap and box tool documentation
- Updated PROGRESS.md: DC-1 testing results and fixes applied
- Updated CLAUDE.md: key conventions for rendering
- Fix toolbar overlap: use clipToBounds on canvas AndroidView to prevent
  it from drawing over the toolbar area

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/src/main/kotlin/net/metacircular/engpad/ui/editor/EditorScreen.kt

@@ -3,6 +3,7 @@ package net.metacircular.engpad.ui.editor
 import androidx.compose.foundation.layout.Column
 import androidx.compose.foundation.layout.fillMaxSize
 import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.ui.draw.clipToBounds
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.LaunchedEffect
 import androidx.compose.runtime.collectAsState
@@ -114,8 +115,9 @@ fun EditorScreen(
         AndroidView(
             factory = { canvasView },
             modifier = Modifier
-                .fillMaxSize()
-                .weight(1f),
+                .fillMaxWidth()
+                .weight(1f)
+                .clipToBounds(),
         )
     }
 }