Drop the append-only versioning doctrine; document the version script

Module APIs and the library's exported ABI are versioned and retired
deliberately: a break ships as a new table version or a new symbol
version node, and consumers judge the versions they are handed.

Document src/libdpm-core.map — what it pins, what belongs in it, and
how a generation is retired — and declare DPM_CORE_0.1 as the shape
the next retired node takes.

src/libdpm-core.map

@@ -1,3 +1,28 @@
+/*
+ * libdpm-core.map — the linker version script for libdpm-core.so
+ *
+ * Passed to the linker with --version-script. It does two things: it
+ * pins the library's exported symbol list, and it stamps every exported
+ * symbol with the version node named below.
+ *
+ * What goes in `global`: every function declared in the public header,
+ * include/dpm/core.h, and nothing else. The library compiles with
+ * hidden default visibility, so `local: *` is what the implementation
+ * already gets; listing it here makes the export set explicit and fails
+ * the link loudly if the two ever disagree.
+ *
+ * Changing it: adding a public function means adding its name here in
+ * the same commit that adds it to the header. Removing or renaming one
+ * breaks every consumer already linked against it, so a removal ships
+ * as a new node above the existing one — binaries already linked
+ * against the older node keep resolving it while new links bind to the
+ * newer one.
+ *
+ * The node name carries the ABI generation. Newest node first, retired
+ * generations below it. DPM_CORE_0.1 at the bottom was never released —
+ * it is there so the next break follows its shape instead of inventing
+ * one.
+ */
 DPM_CORE_1.0 {
     global:
         dpm_open;
@@ -17,3 +42,6 @@ DPM_CORE_1.0 {
     local:
         *;
 };
+
+DPM_CORE_0.1 {
+};