Jackson 2 to 3 migration companion code

Three Maven modules - jackson2-before (2.22.1), jackson3-after (3.2.1) and a
coexistence module with BOTH majors on one classpath - so every claim in the two
migration guides is executed rather than asserted. Paired class names make the
before/after outputs directly diffable via run-all.sh.

Confirms the guides on wire-format equivalence (10-case suite, zero mismatches),
classpath coexistence and the collapse of four artifacts into one. Corrects nine
points, including that enableDefaultTyping() is still present in Jackson 2.22.1
rather than removed in 2.16, and that the published "after" mapper snippet does
not compile.

docs/output/coexistence-S08WireFormatEquivalence.txt

@@ -0,0 +1,14 @@
+plain record       identical
+nested list        identical
+map                identical
+java.time          identical
+Optional present   identical
+Optional empty     identical
+NON_NULL           identical
+@JsonProperty      identical
+@JsonFormat        identical
+nulls and empties  identical
+
+cases     : 10
+mismatches: 0
+Wire format is identical across both majors for these cases.