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/lib.rs

@@ -68,6 +68,10 @@ pub mod localization;
 // Whole-ship assembly from XBG7 part families (capital ships as split parts).
 pub mod ship;
 
+/// The runtime layout of a unit / vessel definition object, read out of the
+/// title's loader and verified against a live mission (see the module docs).
+pub mod unit_layout;
+
 // Exact capital-ship placement from a runtime F10 ship-capture (ground truth).
 pub mod ship_capture;