More progress; broke up "expansion" code into individual components/functions each exercising a distinct function (expand shorthand question-mark syntax, expand shorthand variable substitution, etc).
test/template/: Create new test-file, variables.php, for tests specific to variable substitution; improve 'normalizeSpace' helper-function a bit.
src/template/: Expect variable names to begin with letter or underscore -- i.e., do not consider a string like "50" to be a variable.
test/template/variables.php: Add test-case to assert previous bug-fix/commit works as expected.
test/template/internals.php: Just remove commented-out require statement.
src/template/base.php: A good bit of reorganizing/refactoring; make 'renderFromString' properly handle variables again.
src/template/inheritance.php: Add 'expandBlockReferences' functionality.