build: single source of truth for the version

Version now lives only in [workspace.package] in the root Cargo.toml;
all three crates inherit it via version.workspace = true (this also fixes
gipfel-core, which had drifted to 0.1.0 while the others were 0.5.4).
edition/license are inherited the same way.

The About dialog now reads env!("CARGO_PKG_VERSION") instead of a
hardcoded string. A new test (changelog_top_matches_version) asserts the
top gipfel-X.Y.Z entry in ChangeLog equals the crate version, so the
manifest, the About dialog and the ChangeLog cannot drift apart.

AGENTS.md gains a "Releasing a new version" section documenting the flow
(bump workspace version, add ChangeLog entry, tag, git archive source
tarball).

1 files changed, 23 insertions, 1 deletions

diff --git a/crates/gipfel-app/src/main.rs b/crates/gipfel-app/src/main.rs
index 967e68d..12e2845 100644
--- a/crates/gipfel-app/src/main.rs
+++ b/crates/gipfel-app/src/main.rs
@@ -1526,7 +1526,7 @@ impl Gipfel {
     fn view_about<'a>(&self) -> Element<'a, Message> {
         widget::dialog()
             .title("About gipfel")
-            .body("gipfel 0.5.4")
+            .body(format!("gipfel {}", env!("CARGO_PKG_VERSION")))
             .control(widget::text("Annotate mountain panoramas."))
             .control(widget::text("by Johannes Hofmann"))            .primary_action(widget::button::suggested("Close").on_press(Message::About(false)))
             .is_overlay(false)
@@ -1802,4 +1802,26 @@ mod tests {
         let (_, y_back) = pan.project_point(0.0, top).unwrap();
         assert!((y_back + 100.0).abs() < 0.01);
     }
+
+    /// Release drift guard: the top `gipfel-X.Y.Z` entry in the ChangeLog must
+    /// match the crate version (inherited from `[workspace.package]`). This
+    /// catches a bump of one but not the other. The About dialog reads the same
+    /// `CARGO_PKG_VERSION`, so all three stay in lockstep.
+    #[test]
+    fn changelog_top_matches_version() {
+        let path = concat!(env!("CARGO_MANIFEST_DIR"), "/../../ChangeLog");
+        let text = std::fs::read_to_string(path)
+            .expect("ChangeLog not found at the workspace root");
+        let top = text
+            .lines()
+            .map(str::trim)
+            .find_map(|l| l.strip_prefix("gipfel-"))
+            .expect("ChangeLog has no `gipfel-X.Y.Z` entry");
+        assert_eq!(
+            top,
+            env!("CARGO_PKG_VERSION"),
+            "ChangeLog top entry is `gipfel-{top}` but crate version is {}",
+            env!("CARGO_PKG_VERSION")
+        );
+    }
 }