feat(formats): land the verified unit-definition layout as data + API + test

The offset->field map read out of sub_82341A20 is now usable rather than just
documented: data/unit_definition_layout.txt carries the 159 fields with their
provenance and the two conventions (angles are degrees on disc and radians in the
object; a defaulted field keeps the accessor's 0.0 miss value), and
sylpheed_formats::unit_layout exposes fields()/field_at()/offset_of() so a memory
snapshot can be read by name.

tests/unit_layout_disc.rs replays the verification against the checked-in live
dump -- every mapped float of all 11 identified objects must equal its disc value,
angles compared in radians -- asserting 0 disagreements and >=400 agreements. It
needs no emulator. Full suite green: 11 binaries.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

crates/sylpheed-formats/src/unit_layout.rs

@@ -0,0 +1,105 @@
+//! The runtime layout of a unit / vessel definition object.
+//!
+//! The game parses an [`crate::idxd`] record into a fixed 880-byte object whose
+//! field offsets are **not** guessable from the disc data: the record is a
+//! reflective key/value pool, and the loader assigns each key to a member by
+//! name. This table is that assignment, read out of the loader itself
+//! (`sub_82341A20` — every key is built as `addi r4, r30, -N`, so the field name
+//! for each store is a string in the executable image), and verified against
+//! objects dumped from a running mission: **406 values agree with the disc
+//! records, 0 disagree**, over 11 objects covering both schemas.
+//!
+//! Why a reimplementation wants it:
+//!
+//! - it names the field behind every word of a live definition object, so a
+//!   memory snapshot can be read directly;
+//! - it says which fields a record leaves **defaulted**, and what the loader
+//!   leaves there — the float accessor returns `0.0` on a pool miss;
+//! - it carries two conventions that are invisible on disc: **angles are degrees
+//!   in the data and radians in the object**, and **`Size_Y` takes `Size_X`**
+//!   when omitted.
+//!
+//! See `docs/re/live-unit-definitions.md` for the derivation and the measured
+//! default values.
+
+/// How a field is stored in the definition object.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Kind {
+    /// IEEE-754 single, big-endian.
+    F32,
+    /// Pointer to a string.
+    Str,
+    /// 32-bit word (bool / enum / count / id).
+    Word,
+}
+
+/// One field of the definition object.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct Field {
+    /// Byte offset from the start of the object.
+    pub offset: usize,
+    /// Field name, exactly as the disc record spells it.
+    pub name: &'static str,
+    /// Storage kind.
+    pub kind: Kind,
+}
+
+const TABLE: &str = include_str!("../data/unit_definition_layout.txt");
+
+/// Every mapped field, in offset order.
+pub fn fields() -> Vec<Field> {
+    TABLE
+        .lines()
+        .filter(|l| !l.trim_start().starts_with('#') && !l.trim().is_empty())
+        .filter_map(|l| {
+            let mut it = l.split_whitespace();
+            let offset = it.next()?.parse().ok()?;
+            let kind = match it.next()? {
+                "f32" => Kind::F32,
+                "str" => Kind::Str,
+                _ => Kind::Word,
+            };
+            // `name` is a &'static str because TABLE is 'static.
+            let name = it.next()?;
+            let name: &'static str = TABLE.get(
+                TABLE.find(name).map(|s| s..s + name.len())?,
+            )?;
+            Some(Field { offset, name, kind })
+        })
+        .collect()
+}
+
+/// The field at `offset`, if one is mapped there.
+pub fn field_at(offset: usize) -> Option<Field> {
+    fields().into_iter().find(|f| f.offset == offset)
+}
+
+/// The offset of `name`, if it is mapped.
+pub fn offset_of(name: &str) -> Option<usize> {
+    fields().into_iter().find(|f| f.name == name).map(|f| f.offset)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn table_parses_and_is_ordered() {
+        let f = fields();
+        assert!(f.len() > 150, "expected the full map, got {}", f.len());
+        assert!(f.windows(2).all(|w| w[0].offset < w[1].offset), "offsets must be strictly increasing");
+    }
+
+    #[test]
+    fn known_fields_sit_where_the_loader_puts_them() {
+        // Spot-checks from the verified map; these four also anchor the
+        // identification of a live object (see docs/re/live-unit-definitions.md).
+        assert_eq!(offset_of("Size_X"), Some(48));
+        assert_eq!(offset_of("Size_Y"), Some(52));
+        assert_eq!(offset_of("Size_Z"), Some(56));
+        assert_eq!(offset_of("HP"), Some(84));
+        assert_eq!(offset_of("HQRatio"), Some(88));
+        assert_eq!(field_at(96).map(|f| f.name), Some("ThrusterRatio"));
+        assert_eq!(field_at(48).map(|f| f.kind), Some(Kind::F32));
+    }
+}