Implement uniq: Unique Lines

You are implementing the core behavior of a uniq-style command. Given the lines of a file in order, return only the first occurrence of each distinct line while preserving the original order of first appearance.
The interview follow-up may ask how to handle files too large to fit in memory. For this function, implement the in-memory version over the provided array of lines.

Function
getUniqueLines(lines: String[]) → String[]

Examples
Example 1
lines = ["apple", "banana", "apple", "carrot", "banana"]
return = ["apple", "banana", "carrot"]
Only the first occurrence of each distinct line is kept.
Example 2
lines = ["", "x", "", "X"]
return = ["", "x", "X"]
Empty strings are valid lines, and comparison is case-sensitive.

Constraints
Preserve the order of first appearance.
Line comparison is exact and case-sensitive.
The base function may use memory proportional to the number of distinct lines.

The trick is one pass with a hash set. Walk the array, check whether the line is already in the set, and if not, add it to the set and append it to the result. That's O(n) time and O(d) memory for d distinct lines, which the constraints explicitly allow. The pitfalls are small but real. Don't sort, because that destroys first-appearance order. Don't lowercase or trim anything, since "x" and "X" must stay separate. Don't treat the empty string as falsy and skip it, which bites in JavaScript and Python if you write sloppy truthiness checks. Example 2 tests exactly that. For the memory follow-up, mention hashing each line to a fixed-size digest, or an external sort-based approach, but only after the in-memory version passes. If you freeze mid-assessment, StealthCoder is the safety net that hands you the clean version.

Drill it cold or hedge it with StealthCoder. Either way, don't walk into the OA hoping you remember the trick.