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/e2e/README.md

@@ -0,0 +1,85 @@
+# End-to-End User Journey Tests
+
+This directory contains tests that simulate the complete student experience with TinyTorch.
+
+## Philosophy
+
+Unlike unit tests (test components) or integration tests (test interactions), E2E tests verify:
+- **The complete user journey works from start to finish**
+- **Commands chain together correctly**
+- **Progress tracking persists across operations**
+- **Milestones unlock at the right time**
+
+## Test Levels
+
+### Level 1: Quick Verification (~30 seconds)
+```bash
+pytest tests/e2e/test_user_journey.py -k quick -v
+```
+- Verifies CLI commands work
+- Checks module/milestone structure exists
+- No actual training
+
+### Level 2: Module Flow (~2 minutes)
+```bash
+pytest tests/e2e/test_user_journey.py -k module_flow -v
+```
+- Tests module start → complete → export cycle
+- Verifies progress tracking
+- Tests prerequisite enforcement
+
+### Level 3: Full Journey (~5-10 minutes)
+```bash
+pytest tests/e2e/test_user_journey.py -k full_journey -v
+```
+- Complete flow from setup to first milestone
+- Actual module completion
+- Milestone unlock verification
+
+### Level 4: Release Validation (~30 minutes)
+```bash
+./tests/e2e/validate_release.sh
+```
+- Simulates fresh git clone
+- Full setup through milestone 01
+- Tests pip installability
+- Comprehensive verification
+
+## Running Before Release
+
+Before any release, run:
+
+```bash
+# Quick sanity check
+make e2e-quick
+
+# Full validation (recommended before release)
+make e2e-full
+```
+
+## What Gets Tested
+
+1. **Setup Flow**
+   - `tito setup --skip-profile` works non-interactively
+   - Environment validation passes
+   - Package is importable
+
+2. **Module Workflow**
+   - `tito module start 01` works
+   - `tito module complete 01` exports correctly
+   - Progress tracking updates
+   - `tito module status` shows correct state
+
+3. **Prerequisite Enforcement**
+   - Can't start module 02 without completing 01
+   - Can't run milestone 02 without prerequisites
+
+4. **Milestone Flow**
+   - Milestones list correctly
+   - `tito milestone run 01` executes
+   - Completion is tracked
+
+5. **Error Handling**
+   - Graceful failures for invalid commands
+   - Helpful error messages
+