Add structured exception logging

[rtcoder]

Summary

• add a zero-dependency logging layer with LoggerInterface, NullLogger, JsonFileLogger, and LoggerFactory
• log framework exceptions from App and the global ErrorHandler with structured context
• include request method, path, IP, user agent, request id, status, file, and line in exception log context
• support LOG_ENABLED and LOG_PATH environment configuration
• allow replacing the logger through the service container
• document logging in README, developer docs, .env.example, and refactoring notes

Tests

• composer validate --no-check-publish
• find src application tests -name '*.php' -print0 | xargs -0 -n1 php -l
• composer test
• LOG_ENABLED=true LOG_PATH=/private/tmp/shift-log-smoke.log php -r 'require "bootstrap.php"; (Shift\Logging\LoggerFactory::fromEnv())->log("info", "smoke", ["ok" => true]); echo is_file("/private/tmp/shift-log-smoke.log") ? "log ok\n" : "missing\n";'