WIP: EXIF metadata on export

Checkpoint committed by the coordinator, not by the authoring agent: the
session hit its API limit mid-task and left this work uncommitted. Committed
so it survives, NOT because it is finished - expect failing tests and
half-applied changes. The agent resumes from here.

core/dr-decode/src/lib.rs

@@ -21,8 +21,8 @@ pub mod profile;
 pub use base_curve::BaseCurve;
 pub use error::DecodeError;
 pub use locate::{
-    defects, is_complete_jpeg, locate_preview, BadLine, BadPixel, Defects, PreviewLocation,
-    HEADER_BYTES,
+    defects, is_complete_jpeg, jpeg_metadata, locate_preview, tiff_metadata, BadLine, BadPixel,
+    Defects, PreviewLocation, HEADER_BYTES,
 };
 pub use preview::{
     decode_jpeg, extract_embedded_preview, extract_preview, Preview, PreviewSize,
@@ -30,7 +30,7 @@ pub use preview::{
 };
 pub use profile::CameraProfile;
 
-use dr_types::{Format, Orientation};
+use dr_types::{Format, Location, Orientation};
 
 /// Capture metadata read from a file header.
 #[derive(Debug, Clone, Default, PartialEq)]
@@ -68,6 +68,27 @@ pub struct Metadata {
     /// to where it was taken, so without this a shoot in Tokyo displays on the
     /// wrong day in Paris.
     pub captured_offset: Option<i32>,
+    /// TRACES: FR-EXP-8
+    /// Who made the photograph (EXIF `Artist`, 0x013B).
+    ///
+    /// Read for the sake of exporting it again: a photographer who set a byline
+    /// in-camera set it so that it would still be there in the copy they hand
+    /// over, and an export that dropped it would be quietly removing the one
+    /// piece of metadata that says whose work this is.
+    pub artist: Option<String>,
+    /// TRACES: FR-EXP-8
+    /// The rights statement (EXIF `Copyright`, 0x8298).
+    pub copyright: Option<String>,
+    /// TRACES: FR-EXP-8
+    /// Where the shutter fired (the EXIF GPS directory, 0x8825).
+    ///
+    /// **Read, but treated as radioactive downstream.** This is the field
+    /// FR-EXP-8's strip option exists for, and the export path drops it unless
+    /// the user has explicitly said otherwise (`ExportSettings::strip_location`
+    /// defaults to on). Parsing it here rather than refusing to look is what
+    /// makes "keep my coordinates" possible at all, and what lets the exporter
+    /// prove the field is gone rather than hope it was never present.
+    pub location: Option<Location>,
 }
 
 /// Decoded sensor data, before demosaic.
@@ -302,6 +323,10 @@ pub fn metadata(bytes: &[u8]) -> Result<Metadata, DecodeError> {
             .as_deref()
             .or(exif.offset_time.as_deref())
             .and_then(parse_exif_offset),
+        // TRACES: FR-EXP-8
+        artist: exif.artist.clone().filter(|s| !s.trim().is_empty()),
+        copyright: exif.copyright.clone().filter(|s| !s.trim().is_empty()),
+        location: exif.gps.as_ref().and_then(rawler_location),
     };
 
     // rawler reports no capture time for some TIFF-derived files whose tag is
@@ -322,12 +347,57 @@ pub fn metadata(bytes: &[u8]) -> Result<Metadata, DecodeError> {
             out.iso = out.iso.or(fallback.iso);
             out.lens = out.lens.take().or(fallback.lens);
             out.orientation = out.orientation.or(fallback.orientation);
+            // TRACES: FR-EXP-8
+            // Filled from the same walk for the same reason: a file rawler
+            // answered short on is one whose byline and rights statement would
+            // otherwise be dropped at export, and both sit in the IFD this has
+            // already read.
+            out.artist = out.artist.take().or(fallback.artist);
+            out.copyright = out.copyright.take().or(fallback.copyright);
+            out.location = out.location.or(fallback.location);
         }
     }
 
     Ok(out)
 }
 
+/// TRACES: FR-EXP-8
+/// rawler's GPS directory as a position.
+///
+/// The three-rational form is the tag's, not a position's: degrees, minutes
+/// and seconds, each a fraction, with the hemisphere in a separate letter.
+/// Everything downstream wants a number it can compare and write back, so the
+/// conversion happens once, here.
+fn rawler_location(gps: &rawler::exif::ExifGPS) -> Option<Location> {
+    /// A `Rational` as a float, with a zero denominator refused rather than
+    /// divided by — some bodies write `0/0` into an unfilled slot.
+    fn ratio(r: &rawler::formats::tiff::Rational) -> Option<f64> {
+        (r.d != 0).then(|| r.n as f64 / r.d as f64)
+    }
+
+    fn degrees(dms: &[rawler::formats::tiff::Rational; 3], reference: Option<&String>) -> Option<f64> {
+        let d = ratio(&dms[0])? + ratio(&dms[1])? / 60.0 + ratio(&dms[2])? / 3600.0;
+        // South and west are stored as positive magnitudes with a letter.
+        let negative = matches!(
+            reference.map(|s| s.trim().to_ascii_uppercase()).as_deref(),
+            Some("S") | Some("W")
+        );
+        Some(if negative { -d } else { d })
+    }
+
+    let latitude = degrees(gps.gps_latitude.as_ref()?, gps.gps_latitude_ref.as_ref())?;
+    let longitude = degrees(gps.gps_longitude.as_ref()?, gps.gps_longitude_ref.as_ref())?;
+    // Reference 1 means below sea level; the altitude itself is unsigned.
+    let altitude = gps.gps_altitude.as_ref().and_then(ratio).map(|a| {
+        if gps.gps_altitude_ref == Some(1) {
+            -a
+        } else {
+            a
+        }
+    });
+    Location::new(latitude, longitude, altitude)
+}
+
 /// TRACES: FR-CAT-5 | FR-DEV-3h
 /// Read just the stored orientation, from a file header.
 ///