refactor(hm-dsl-engine): Replace positional (bool, bool) detection result with a named struct or DslLanguage set

[markovejnovic]

The smell

scan_extensions in crates/hm-dsl-engine/src/detect.rs returned a positional (bool, bool) meaning (has_py, has_ts). Three in-file consumers destructured it positionally:

• detect_language matched (_, true) / (true, false)
• detect_language_python_first matched (true, _) / (false, true)
• has_pipeline_files bound Ok((py, ts))

The two booleans are the same type, so swapping them (or transposing a match arm) is a silent logic bug the compiler cannot catch — a Python repo could resolve to TypeScript and still type-check.

The change

scan_extensions now returns a tiny named struct:

struct DetectedLangs {
    has_py: bool,
    has_ts: bool,
}

All three call sites read .has_py / .has_ts by name. The match-on-tuple sites became if/else if/else chains over the named fields. Logic, ordering (TS-preferred vs Python-preferred), error messages, and the public API (detect_language, detect_language_python_first, has_pipeline_files) are all unchanged. The new struct is private to the module.

Type-safety pattern

This applies the "relational filter / result as a named type, not a loose bool pair" guidance from fd (sharkdp/fd): naming the fields removes the positional-swap hazard while keeping behavior and the public surface identical. A has_py/has_ts mix-up is now impossible to express because the fields are addressed by name rather than position.

Validation

Only cargo check -p hm-dsl-engine was run locally (passes clean). Full CI runs on this PR. Opened as draft for review.