Fix CI typecheck errors: widen Index generic signatures and handle noUncheckedIndexedAccess

[Copilot]

PR #4 introduced the Index<T> type system but CI fails with 27 typecheck errors across two categories.

Handle undefined from noUncheckedIndexedAccess

Array index access returns T | undefined under strict config. Added explicit undefined checks in isMonotonicIncreasing, isMonotonicDecreasing, and argsort comparators.

Widen method type signatures

Methods like union, intersection, append, insert, fillna were typed as (other: Index<T>): Index<T>, making it impossible to operate across indices with different literal element types:

const a = new Index([1, 2, 3]);    // Index<1 | 2 | 3>
const b = new Index([3, 4, 5]);    // Index<3 | 4 | 5>
a.union(b);  // TS2345: Index<3|4|5> not assignable to Index<1|2|3>

Changed to second type parameter where the result combines both types:

• Set ops (union, intersection, symmetricDifference): <U extends Label>(other: Index<U>): Index<T | U>
• difference: <U extends Label>(other: Index<U>): Index<T> — only returns self's elements
• append, insert, fillna: <U extends Label>(...): Index<T | U>
• equals, identical: accept Index<Label>
• getLoc, contains, getIndexer: accept Label / Index<Label>