feat: integrate TinyTorch into MLSysBook repository

TinyTorch educational deep learning framework now lives at tinytorch/

Structure:
- tinytorch/src/         - Source modules (single source of truth)
- tinytorch/tito/        - CLI tool
- tinytorch/tests/       - Test suite
- tinytorch/site/        - Jupyter Book website
- tinytorch/milestones/  - Historical ML implementations
- tinytorch/datasets/    - Educational datasets (tinydigits, tinytalks)
- tinytorch/assignments/ - NBGrader assignments
- tinytorch/instructor/  - Teaching materials

Workflows (with tinytorch- prefix):
- tinytorch-ci.yml           - CI/CD pipeline
- tinytorch-publish-dev.yml  - Dev site deployment
- tinytorch-publish-live.yml - Live site deployment
- tinytorch-build-pdf.yml    - PDF generation
- tinytorch-release-check.yml - Release validation

Repository Variables added:
- TINYTORCH_ROOT  = tinytorch
- TINYTORCH_SRC   = tinytorch/src
- TINYTORCH_SITE  = tinytorch/site
- TINYTORCH_TESTS = tinytorch/tests

All workflows use \${{ vars.TINYTORCH_* }} for path configuration.

Note: tinytorch/site/_static/favicon.svg kept as SVG (valid for favicons)

tinytorch/tests/regression/README.md

@@ -0,0 +1,146 @@
+# TinyTorch Regression Tests
+## Ensuring Core Infrastructure Works Correctly
+
+This directory contains regression tests that ensure TinyTorch's core functionality works correctly so students don't get stuck on infrastructure issues.
+
+---
+
+## 📋 Test Coverage
+
+### Shape Compatibility Tests
+**File**: `test_conv_linear_dimensions.py`
+**What it tests**: Convolution output dimensions match Linear layer expectations
+**Why it matters**: Students shouldn't debug dimension mismatches in their CNNs
+
+### Tensor Reshaping Tests  
+**File**: `test_transformer_reshaping.py`
+**What it tests**: Transformer 3D outputs work with Linear 2D layers
+**Why it matters**: Language model architectures should "just work"
+
+---
+
+## 🧪 Running Regression Tests
+
+### Run All Regression Tests
+```bash
+pytest tests/regression/
+```
+
+### Run Specific Bug Test
+```bash
+pytest tests/regression/test_issue_20241125_conv_fc_shapes.py -v
+```
+
+### Run with Coverage
+```bash
+pytest tests/regression/ --cov=tinytorch --cov-report=html
+```
+
+---
+
+## 📝 Adding New Regression Tests
+
+When you discover a bug:
+
+1. **Create Test File**: `test_issue_YYYYMMDD_description.py`
+
+2. **Use Bug Tracking Template**:
+```python
+"""
+BUG TRACKING:
+============
+Bug ID: BUG-YYYY-MM-DD-XXX
+Date Found: YYYY-MM-DD
+Found By: [Name/System]
+Severity: [Critical/High/Medium/Low]
+
+DESCRIPTION:
+[What broke and under what conditions]
+
+REPRODUCTION:
+[Exact steps to reproduce]
+
+ROOT CAUSE:
+[Why it happened]
+
+FIX:
+[What was changed to fix it]
+
+PREVENTION:
+[How this test prevents recurrence]
+"""
+```
+
+3. **Write Specific Test**: Test the EXACT scenario that failed
+
+4. **Verify Test Catches Bug**: 
+   - Test should FAIL without the fix
+   - Test should PASS with the fix
+
+5. **Update This README**: Add entry to Bug Index
+
+---
+
+## 🎯 Testing Philosophy
+
+**Every bug tells a story about a gap in our testing.**
+
+When we find a bug, we ask:
+1. Why didn't existing tests catch this?
+2. What test would have prevented it?
+3. Are there similar bugs we haven't found yet?
+
+**The goal**: Build a test suite so comprehensive that bugs become impossible.
+
+---
+
+## 📊 Regression Test Statistics
+
+- **Total Bugs Found**: 2
+- **Bugs with Regression Tests**: 2 (100%)
+- **Test Coverage**: 100% of discovered issues
+- **Last Updated**: 2024-11-25
+
+---
+
+## 🔄 Integration with CI/CD
+
+These regression tests run automatically on:
+- Every commit to main branch
+- Every pull request
+- Nightly comprehensive test suite
+
+Failures in regression tests block deployment to ensure fixed bugs never return.
+
+---
+
+## 🏆 Success Metrics
+
+We measure success by:
+1. **Zero Regressions**: No bug returns after being fixed
+2. **Fast Detection**: Regression tests catch issues immediately
+3. **Clear Documentation**: Every test explains the bug it prevents
+4. **Continuous Growth**: New bugs always get new tests
+
+---
+
+## 📚 Learning from Bugs
+
+Each bug teaches us something:
+
+- **Conv Shape Mismatch**: Always calculate dimensions programmatically, never manually
+- **Transformer Reshape**: Consider tensor dimensionality at module boundaries
+- **[Future bugs will add lessons here]**
+
+---
+
+## 🚀 Future Improvements
+
+- [ ] Add performance regression tests
+- [ ] Create fuzz testing for edge cases
+- [ ] Build automatic bug report generation
+- [ ] Implement regression test metrics dashboard
+
+---
+
+Remember: **A bug fixed without a test is a bug waiting to return.**