ChainSel.ReprocessLoEBlocks: avoid transient chain switches (#1617)

Closes #1616

The commits are intended to be reviewed individually.

- The first seven commits are preparatory refactorings of the existing
logic; they (intentionally) do not fix the bug, and actually might be of
independent interest, breaking up the huge `chainSelectionForBlock`
function.

Using structural diffing via eg
[difftastic](https://github.com/Wilfred/difftastic) might be useful
while reviewing.

 - The last commit fixes the bug as described in #1616.

Author: amesgen

ouroboros-consensus/changelog.d/20250801_172359_alexander.esgen_chainsel_fix_transient_loe.md

@@ -0,0 +1,5 @@
+### Breaking
+
+- Changed `SelectionChangedInfo.newTipTrigger` (contained in tracing types) to
+  be a `Maybe` to accurately reflect Ouroboros Genesis-related (Limit on
+  Eagerness) triggered chain selection.