Rename ResolveSameDir to ResolveSamePath and fix destination error handling

[Claude]

The ResolveSameDir error was misleading since it's returned for both same-file and same-directory detection. Additionally, the destination stat error handler incorrectly returned SourceLocationInvalid when the destination file type was unsupported.

Changes

• Renamed error variant: ResolveSameDir → ResolveSamePath across all error sites in src/file.zig, src/copy.zig, and src/main.zig

• Added DestLocationInvalid error: New variant in ResolveError set with corresponding handler in src/main.zig that reports unsupported destination file types correctly

• Fixed destination stat error: Changed src/file.zig:109 to return DestLocationInvalid instead of SourceLocationInvalid when destination file kind is not supported

• Backend dispatch improvement: Replaced three sequential if checks followed by unreachable with exhaustive switch statement in src/main.zig, ensuring compiler-enforced exhaustiveness

// Before
if (options.backend == .evented) { ... }
if (options.backend == .single) { ... }
if (options.backend == .threaded) { ... }
unreachable;

// After
switch (options.backend) {
    .evented => { ... },
    .single => { ... },
    .threaded => { ... },
}