Improve Dtype + Series: widened signatures, lint fixes, and removed unsafe casts

[Copilot]

The Dtype + Series code from Autoloop Iteration 3 was already merged to main via PR #15. This PR adds improvements on top of that merge.

Lint/type fixes

• Routed exports through barrel (src/core/index.ts) to satisfy useImportRestrictions
• Added exhaustive default clause to Dtype.build() switch
• Extracted compareScalars() helper to bring sortValues under cognitive complexity limit
• Widened arithmetic/comparison/isin signatures from T | Series<T> to number | Series<Scalar> / Scalar | Series<Scalar>, eliminating all as never casts in tests:

// Before: literal type narrowing forced unsafe casts
const s = new Series({ data: [1, 2, 3] }); // Series<1 | 2 | 3>
s.add(10 as never); // required cast

// After: widened signatures accept any number
s.add(10); // works directly

Conflict resolution

• Merged origin/main into branch to resolve conflicts, keeping our improved versions

[mrjf]

@copilot was this already merged? if not resolve conflicts

[Copilot]

@copilot was this already merged? if not resolve conflicts

PR #15 already merged the Dtype + Series to main. I've now merged origin/main back into this branch to resolve the conflicts (commit 9cc05c8). The remaining diff contains only improvements on top of what landed: widened method signatures, barrel exports, exhaustive switch default, extracted sort comparator, and removed as never casts from tests. All 218 tests pass.