Fix column-span clipping for indexed maps with line offsets

[NUAAwqy]

Indexed maps with a nonzero section line offset currently report an infinite column span where the following section starts on the same generated line. This can make a mapping's reported range include columns owned by the next section.

Apply the line offset before checking the next section's boundary in generatedPositionFor, allGeneratedPositionsFor, and eachMapping. Keep the first-line column-offset adjustment in section-local coordinates.

Add 12 regression and boundary tests covering all three APIs, both the first and later section lines, a next section on a different line, finite spans, and uncomputed terminal spans.

Closes #539

Validation (Node.js 22.22.0):

• Original implementation: 9/12 new tests pass; one regression fails for each affected API.
• Fixed implementation: 12/12 new tests and 202/202 full tests pass.
• npm run coverage, repository-wide ESLint, and Prettier on the new test file pass.
• The full suite uses the repository's pinned tc39/source-map-tests revision.

Implementation and test execution were assisted by Codex.