docs: Update plan and progress for subtask 3.5 completion

.auto-claude/specs/001-conceptual-explanations/build-progress.txt

@@ -168,3 +168,98 @@ Implementation details:
 
 Committed: 3c08b45
 Status: ✓ Completed
+
+=== 2026-01-11 - Subtask 3.2 Completed ===
+Added conceptual explanations to all 6 CSS Grid lessons.
+
+Implementation details:
+- Added 'concept' objects to all Grid lessons explaining the 2D grid system, tracks, and cell placement
+- Lesson 1 (Grid Container Basics): 
+  * Explanation of 2D layout system, tracks (rows/columns), 1fr units, and gap property
+  * Diagram showing grid container with 3 equal columns and 2 rows
+  * Container vs Item: display: grid, grid-template-columns, and gap are container properties
+- Lesson 2 (Grid Template Areas):
+  * Explanation of ASCII-art layouts and named grid areas for spanning
+  * Diagram showing visual layout with header, sidebar, content, footer regions
+  * Container vs Item: grid-template-areas (container) vs grid-area (item)
+- Lesson 3 (Spanning Grid Cells):
+  * Explanation of spanning multiple cells with grid-column/grid-row span keyword
+  * Diagram showing 2x2 spanning featured item with auto-flow around it
+  * Container vs Item: grid-column and grid-row are item properties
+- Lesson 4 (Automatic Grid Placement):
+  * Explanation of auto-fit with minmax for responsive grids without media queries
+  * Diagram comparing wide vs narrow viewport behavior
+  * Container vs Item: grid-template-columns with auto-fit is a container property
+- Lesson 5 (Grid Alignment):
+  * Explanation of justify-items (horizontal) and align-items (vertical) alignment
+  * Diagram showing items centered within grid cells on both axes
+  * Container vs Item: justify-items/align-items (container) can be overridden by justify-self/align-self (item)
+- Lesson 6 (Overlapping Grid Items):
+  * Explanation of overlapping items in same cell using explicit positioning and z-index
+  * Diagram showing layered items with z-index stacking
+  * Container vs Item: grid-column, grid-row, and z-index are item properties
+- All explanations are beginner-friendly, 2-4 sentences
+- ASCII diagrams provide visual understanding of grid concepts
+- Clear distinction between container and item properties throughout
+
+Committed: 29c019b
+Status: ✓ Completed
+
+=== 2026-01-11 - Subtask 3.3 Completed ===
+Added conceptual explanations for CSS selector specificity and cascade.
+
+Implementation details:
+- Added 'concept' objects to 4 lessons in lessons/00-basic-selectors.json
+- Lesson 7 (Type + ID): Explains specificity boost from combining type and ID selectors
+  * Shows how p#special has higher specificity than #special alone
+  * Diagram demonstrates both conditions must match (type AND id)
+  * Emphasizes enforcement pattern for IDs on specific element types
+- Lesson 8 (Selector Lists): Explains OR logic and independent matching
+  * Shows how comma-separated selectors are treated independently
+  * Diagram demonstrates each selector matches separately
+  * Clarifies that selectors maintain individual specificity
+- Lesson 9 (Universal Selector): Explains wildcard matching and descendant context
+  * Shows how * matches all element types
+  * Diagram demonstrates descendant relationship with space
+  * Explains difference between global * and contextual .container *
+- Lesson 10 (Specificity): Explains CASCADE and specificity point system
+  * Introduces point system: IDs=100, classes=10, elements=1
+  * Diagram shows specificity calculation with example selectors
+  * Demonstrates how higher specificity wins the cascade
+- All explanations are beginner-friendly, 2-4 sentences
+- ASCII diagrams provide visual understanding of selector matching and cascade resolution
+- Focuses on WHY certain selectors match and HOW conflicts are resolved
+
+Committed: 39f1fb5
+Status: ✓ Completed
+
+=== 2026-01-11 - Subtask 3.5 Completed ===
+Added conceptual explanations to advanced selectors (02-selectors.json).
+
+Implementation details:
+- Added 'concept' objects to all 4 lessons explaining advanced selector concepts
+- Lesson 1 (Element Selectors):
+  * Explanation of DOM traversal and how browser matches tag names
+  * ASCII diagram showing browser checking each element type
+  * Specificity: 0,0,0,1 (lowest - easy to override)
+- Lesson 2 (Class Selectors):
+  * Explanation of attribute matching independent of element type
+  * Diagram showing class matching across different element types
+  * Specificity: 0,0,1,0 (10x stronger than elements)
+- Lesson 3 (ID Selectors):
+  * Explanation of unique ID matching and high specificity
+  * Diagram showing single match and specificity comparison table
+  * Specificity: 0,1,0,0 (100x stronger than classes)
+  * Explains why developers prefer classes over IDs
+- Lesson 4 (Combined Selectors):
+  * Explanation of AND logic (no space between selectors)
+  * Diagram showing both conditions must match
+  * Specificity addition: div.note = 0,0,1,1 beats .note = 0,0,1,0
+  * Emphasizes how cascade resolves conflicts with specificity
+- All explanations are beginner-friendly (2-4 sentences)
+- ASCII diagrams provide visual understanding of selector matching
+- Focus on WHY selectors work and HOW specificity cascade resolves conflicts
+- Explains the fundamental CSS specificity point system throughout
+
+Committed: 3df98fe
+Status: ✓ Completed