CORE-002: bayan-units — integer layout units and deterministic math

Context

ADR-0005 defines the Bayan Layout Unit (BLU = 1/1,828,800 inch = 1/25,400 point) as the integer unit of all layout arithmetic, and requires deterministic math for anything else. This crate is the foundation every other core crate uses for geometry.

Objective

A small, exhaustively tested crate providing BLU, exact unit conversions, explicit rounding, geometry types and deterministic math.

Scope

In scope

• Blu(i64) with arithmetic operators (checked in debug builds, with explicit checked_*/saturating_* variants), ordering, hashing, Display and serde support.
• Exact constants and constructors/accessors for every unit in ADR-0005’s table (inch, point, twip, half-point, eighth of a point, EMU, millimetre, centimetre, pixels at common DPIs).
• Parsing of OOXML measurements: integer twips, EMUs, half-points, eighths of a point, fiftieths of a percent and thousandths, sixty-thousandths of a degree, and Strict OOXML universal measures with units ("12pt", "1.5in", "2cm", "10mm", "3pc", "4pi"), rejecting anything malformed.
• Rational scaling helper: scale(value, numerator, denominator, Rounding) computed in 128-bit integers, with a Rounding enum (floor, ceiling, half-up, half-even, half-away-from-zero, toward zero). This is what the font-unit-to-BLU conversion will use.
• Geometry: Point, Size, Rect, Insets, and a fixed-point affine Transform suitable for rotations and scaling of drawing objects.
• Deterministic math: sine and cosine for transforms (fixed-point or via the pure-Rust libm crate), with tests showing identical results on all targets.
• Property-based tests (proptest): conversion round trips are exact; rounding modes match a big-integer reference implementation; arithmetic does not overflow for any realistic document dimension.

Out of scope

Any text or layout logic.

Deliverables

The crate with documentation and examples; tests.

Acceptance criteria

• AC-1 Every conversion in ADR-0005’s table is exact and covered by a test.
• AC-2 Property-based tests pass, including on wasm32 (via wasm-bindgen-test or Node).
• AC-3 The public API exposes no floating-point types except an explicitly named lossy conversion for display and debugging.
• AC-4 Deterministic math functions return identical bit patterns on Linux x86-64, Windows x86-64, macOS arm64 and wasm32 for a fixed test vector (CI evidence).
• AC-5 cargo xtask verify passes.

Verification

cargo xtask verify; CI matrix results for AC-4.

Escalate if

A conversion required by OOXML cannot be represented exactly in BLU (contradicting ADR-0005).