Add GetItem/SetItem methods and an Items[Key: K] default property to
both TDictionary<K,V> and TOrderedDictionary<K,V>. This enables the
idiomatic d[key] syntax for reads and writes:
D['one'] := 1;
WriteLn(D['one']);
GetItem halts on missing key (consistent with index-out-of-bounds in
TList). SetItem delegates to Add (add-or-update semantics).
The compiler already had full default property infrastructure (parser,
AST, semantic, both codegens) — only the stdlib declarations were
missing. This is the first non-Integer-typed default property in the
codebase, exercising the generic index type path end-to-end.
Tests: 2 IR codegen tests (GetItem/SetItem emission) + 3 e2e tests
on both backends (integer keys, string keys, update-via-bracket).
3407 tests pass. All three fixpoints verified.
a82269f48c