docs(grpc): Add comprehensive documentation explaining why we use the fork

Adds WHY_USE_OPEN_RUNTIME_FORK.md (799 lines) that provides:

CONTEXT:
- Detailed explanation of ServerInterceptor dependency
- Complete mapping of codebase locations that depend on fork features
- Links to specific lines in both fork and monorepo code
- Production impact analysis with/without fork fixes

CRITICAL DEPENDENCIES DOCUMENTED:
1. ServerInterceptor class for security architecture
   - EnhancedConnectionRejectionServerInterceptor (server.dart:1443-1540)
   - Connection rejection tracking system (server.dart:775-1440)
   - Progressive delay implementation (server.dart:1021-1070)

2. Race condition fixes for production stability
   - Safe error handling (handler.dart:318-326)
   - Safe trailer sending (handler.dart:404-410)
   - Safe stream cleanup (handler.dart:442-450)

3. Null connection exception fix
   - Clear error messages (http2_connection.dart:190-193)
   - Defensive connection handling

4. Monorepo path dependencies
   - Required by Melos architecture
   - All packages reference ../../external_dependencies/grpc

USAGE EXAMPLES:
- Cloud Run server initialization (process.dart:723-732)
- JIT test server setup (test files)
- Connection tracking tests

COMPARISON MATRIX:
- Fork vs pub.dev feature availability
- Code locations that would break without fork
- Production impact scenarios
- Architecture dependency map

This documentation serves as reference for:
- Onboarding new developers
- Justifying fork maintenance cost
- Planning upstream sync strategy
- Understanding production requirements

Links to:
- Fork commit URLs
- Monorepo file locations with line numbers
- Upstream references
- Related documentation

Author: tsavo-at-pieces