Fix declarationMap panic on cross-file reused nodes

[Copilot]

During declaration emit with declarationMap enabled, nodes reused from other source files retain positions from their original file. When the printer emits source maps, it indexes into the current (shorter) file's text with those positions, causing a slice bounds panic. JavaScript handles this implicitly since out-of-bounds charCodeAt returns NaN, but Go panics.

• internal/scanner/scanner.go: Bounds check in SkipTriviaEx — return pos unchanged when pos >= len(text)
• internal/printer/printer.go: Bounds check in emitPos — skip source map emission when pos > len(sourceText)
• Test case: declarationMapReusedNodeFromDifferentFile.ts — reproduces the exact crash scenario where a helper file's node positions exceed the shorter index file's text length

// @declaration: true
// @declarationMap: true

// @filename: helper.ts
// ... long file with positions past index.ts length ...
export function getWidget(): (w: Widget) => keyof Widget { ... }

// @filename: index.ts
// short file — reused nodes from helper.ts have out-of-range positions
export const fn = getWidget();

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[DanielRosenwasser]

This is not a good fix. You can only emit a position if a given node belongs to the containing tree, and that's not always the case.

[jakebailey]

This is wrong. #3274