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).

4 files changed, 32 insertions, 10 deletions

diff --git a/crates/gipfel-app/Cargo.toml b/crates/gipfel-app/Cargo.toml
index 00cf451..f16f1c0 100644
--- a/crates/gipfel-app/Cargo.toml
+++ b/crates/gipfel-app/Cargo.toml
@@ -1,9 +1,9 @@
 [package]
 name = "gipfel-app"
-version = "0.5.4"
-edition = "2021"
+version.workspace = true
+edition.workspace = true
 description = "gipfel - photogrammetry for mountain images"
-license = "GPL-2.0-or-later"
+license.workspace = true
 
 [[bin]]
 name = "gipfel"
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")
+        );
+    }
 }
diff --git a/crates/gipfel-cli/Cargo.toml b/crates/gipfel-cli/Cargo.toml
index 0a07982..b1369fa 100644
--- a/crates/gipfel-cli/Cargo.toml
+++ b/crates/gipfel-cli/Cargo.toml
@@ -1,9 +1,9 @@
 [package]
 name = "gipfel-cli"
-version = "0.5.4"
-edition = "2021"
+version.workspace = true
+edition.workspace = true
 description = "gipfel command line tools (export-position, export-hills)"
-license = "GPL-2.0-or-later"
+license.workspace = true
 
 [[bin]]
 name = "gipfel-cli"
diff --git a/crates/gipfel-core/Cargo.toml b/crates/gipfel-core/Cargo.toml
index d8ddfd9..6a66516 100644
--- a/crates/gipfel-core/Cargo.toml
+++ b/crates/gipfel-core/Cargo.toml
@@ -1,9 +1,9 @@
 [package]
 name = "gipfel-core"
-version = "0.1.0"
-edition = "2021"
+version.workspace = true
+edition.workspace = true
 description = "Port of gipfel's computational core (hills, panorama, projections, least-squares fit, metadata)"
-license = "GPL-2.0-or-later"
+license.workspace = true
 
 [dependencies]
 kamadak-exif.workspace = true