FOLLOW-UP: Test coverage v2 for include preprocessing (from PR #256)

[adnaan]

Context

PR #256 shipped 28 unit tests + 3 chromedp e2e + 1 integration test. Several gaps surfaced during review.

Source PR: #256
Suggested by: @claude[bot]
Comment type: PR review (rounds 5, 8)

Task Description

Add the following:

1. region= + lines= mutual-exclusion warning assertion — behavior described in code comments and docs but no test asserts the warning log fires when both are set.
2. FindRegion out-of-order markers — test where the end-marker appears before the start-marker in the file.
3. Symlink-as-escape-vector E2E — TestResolve_PathConfinement covers ../ traversal but not a symlink-as-included-dir pointing outside root. Low risk (EvalSymlinks handles it) but explicit test is good hygiene.
4. data-line-offset arithmetic unit test — currently verified only via E2E; a unit test on the start - 1 math would be tighter, especially when highlight= spans a multi-line range.
5. Concurrent SetExtraFiles stress — locking is correct but no concurrent stress test.

Original Comments

Missing unit test: no case for the region= + lines= mutual-exclusion warning path.
— @claude[bot], round 8

Test coverage gaps: region+lines warning, out-of-order markers
— @claude[bot], round 8

Symlink edge cases: TestResolve_PathConfinement covers ../ traversal but not a symlink-based escape.
— @claude[bot], round 5

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