Align the QBE backend unit names with the dotted naming convention
already used by the native backend (blaise.codegen.native.*).
uCodeGen.pas → blaise.codegen.pas
uCodeGenQBE.pas → blaise.codegen.qbe.pas
Updated all uses clauses (~70 test files, main compiler units),
documentation references (README + 5 docs/*.adoc files), and
comment references in runtime/stdlib.
Captures the rule of thumb that every AST shape change must touch
three places (uAST.pas / uSemantic.pas / uUnitInterfaceIO.pas), with
codegen as a fourth when the node has runtime semantics.
Worked example walks through adding a hypothetical 'when' statement
end-to-end: parser, semantic, QBE codegen, .bif encoder/decoder, and
test surface. Plus a shorter checklist for plain field additions,
common-pitfalls section, and the rationale behind the positional
(rather than tagged) .bif format.
