Support unnamed -> named promotion for return tuples as well.

`def float(...)`, `def int(...)`, `def double(...)`, `def do(...)` were
rejected with error[30163] "invalid function name" — surprising next to
`def string(...)` and `def float2(...)`, which compiled fine. The parser
accepts all of them (function_name → das_type_name); a post-parse lint
guard was the gate, rejecting any name in a hardcoded C++-keyword set.
The asymmetry was just that set: `float` / `int` / `double` are C++
keywords, but `float2` / `string` / `int2` aren't.

Drop the function-name guard and have the AOT emitter mangle keyword
names the same way it already mangles operator overloads.

- daslib/aot_cpp.das: aotSuffixNameEx now seeds prefix = is_cpp_keyword,
  so `def float` AOTs to `_Funcfloat_<hash>` (valid C++) instead of
  literal `float`.
- src/ast/ast_lint.cpp: remove isValidFunctionName + its callsite. The
  other 5 isCppKeyword guards (module/enum/enum-value/struct/field)
  stay — those embed 1:1 into C++ and need the guard.
- tests/language/failed_reserved_names.das: drop the `def do` case and
  30163 from the expect line.
- tests/aot/test_cpp_keyword_names.das: positive test exercising
  `def float`/`def int`/`def double`/`def do` via interpreter, AOT,
  and JIT.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

…eyword-function-names

infer/aot: allow C++-keyword function names via AOT name-mangling

…amed-tuple-conv

compiler: allow unnamed -> named tuples conversions on name match