feat(merge): preserve the user's view across a merge refresh

A merge refresh (the "merge keeping edits" choice on a dirty buffer, or pearl-refresh-current-view on a clean one) used to call pearl--restore-page-visibility at the tail, re-folding the whole buffer to its #+STARTUP overview. That collapsed every subtree the user had expanded, including the comment they were just editing, and disturbed point. The edit-then-merge flow felt jarring.

The fix: pearl--merge-issues-into-buffer now returns a marker for every subtree it re-rendered or appended, alongside the existing counts (:touched-markers (M1 M2 ...)). The merge call sites fold just those subtrees via a new pearl--fold-touched-subtrees helper. Kept (locally-edited) and untouched subtrees stay exactly as the user had them, and point is preserved because the localized fold doesn't touch the rest of the page.

The full-rebuild paths (Branch A no-buffer, Branch B clean-buffer replace, Branch C-discard) still call pearl--restore-page-visibility. They re-render the whole buffer so the global re-fold is the right behavior.

Pre-commit review caught a critical correctness bug in the first draft: the alist marker from pearl--issue-subtree-markers is insertion-type t (it needs to advance past replaces of EARLIER subtrees so it stays anchored to its own heading), and reusing it after pearl--replace-issue-subtree-at-point captured the post-advance position, which is the NEXT subtree's heading, so the fold collapsed the wrong subtree. The fix copies the marker as type-nil BEFORE the replace, so it stays anchored at the deletion start and lands on the new heading after the re-insert. I strengthened the marker test to verify the IDs at the touched markers match the expected set ({a, b, c} for 2 updated + 1 added), not just that they're some valid headings. The original test would have passed against the bug.

Five new tests in test-pearl-merge.el cover: merge returns markers with the correct IDs at their headings, skipped (locally-edited) issues don't add to touched-markers, fold-subtree-at-marker hides body content while leaving the heading visible, fold-touched-subtrees is a no-op when pearl-fold-after-update is nil, and fold-subtree-at-marker tolerates a nil marker silently. All 674 ert tests pass. make compile and make lint are clean.

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