Fix out-of-bounds molidx searches silently returning the last molecule

[lohedges]

This PR closes #286, fixing a bug where out-of-bounds molidx searches silently returned the last molecule instead of raising a KeyError. The root cause was in IDIndexEngine::searchMolIdx(): unlike searchIdx<T> (used for atoms, residues, etc.), it had no single-value fast path and instead fell through to a match() call that used slice.begin(count, true) with auto_fix=true, silently clamping any out-of-bounds index to the last valid position. The fix adds a _is_single_value fast path to searchMolIdx that reads the raw index directly from RangeValue::start (bypassing the _to_single_value helper, which corrupts negative indices by mapping them against INT_MAX) and applies strict Python-style bounds checking. Negative indices within range (e.g. -1) continue to work correctly, while out-of-bounds values in either direction now return no match, consistent with residx and atomidx behaviour. A secondary bug in MolIdx::map(const Molecules &) where the loop counter i was never decremented is also fixed. A regression test is included.

Debugged with assistance from Claude Code Opus 4.6.

@chryswoods: Just want to address you concerns about different container types and use case. In the C++ layer we have MolIdx::map(const Molecules&), where Molecules is a QHash<MolNum, ViewsOfMol>, so has no stable iteration order. I think this isn't an issue for user-facing code, since it goes through another pathway, but good to check. The comment referencing static map() in the issue thread, this pathway isn't touched by this fix, so was a red-herring, i.e. that's used for ranges and comparisons only. For the additional bug fix: I don't think this was ever an issue since this low-level overload probably wasn't used by any callers.

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