docs: fix stale README + guides — plural factories, V3 object-init, real PhysicalConstants surface

[matt-edmondson]

Summary

Pure docs sync — no code or generated output changes. Fixes three user-facing docs that drifted from the actual generator output. Independent of #75 (which only touches *.cs/*.json/Generated/CLAUDE.md), so they should land cleanly together.

What was wrong

1. README.md had aspirational PhysicalConstants.Conversion.X<T>() accessors that were never emitted. CLAUDE.md was cleaned up for this in feat(generator): SEM002 metadata validation + refresh stale generator output #70 but README wasn't.
2. README.md, docs/physics-domains-guide.md, docs/complete-library-guide.md showed V3 construction via 3-arg Force3D<double>.FromNewtons(0.0, 0.0, -9.8) factories that don't exist — generated VN types only have Zero/One/UnitX/Y/Z static properties and object-initializer construction (new Force3D<double> { X = 0.0, Y = 0.0, Z = -9.8 }).
3. docs/complete-library-guide.md had mass * Acceleration<double>.FromMeters(9.8) which is wrong on two counts: Acceleration is a dimension name (the generated V0 base type is AccelerationMagnitude), and Meters is the wrong unit for acceleration (MetersPerSecondSquared).
4. README's disp3d.Magnitude() comment said it returned Distance — but Magnitude() returns the V0 base of the dimension (Length), not a semantic overload.

What changed

• README.md — V3 examples switched to new T<double> { X, Y, Z }. PhysicalConstants examples replaced with the real domain-grouped + Generic<T>() surface. Architecture section now lists units.json / magnitudes.json / conversions.json / domains.json alongside dimensions.json, and the SEM001/SEM002/SEM003 diagnostic line-up. Construction-time invariants from Vector0 quantities do not enforce the non-negativity invariant #50/Domain-specific physical constraints (absolute zero, non-negative frequency, etc.) not enforced #51 added as a code comment.
• docs/physics-domains-guide.md — Magnitude / Cross / Dot examples switched to object-initializer construction.
• docs/complete-library-guide.md — same V3 fix; corrected the bogus Acceleration<double>.FromMeters(9.8) to AccelerationMagnitude<double>.FromMetersPerSecondSquared(9.8); Magnitude() comment corrected to Length not Distance.

Test plan

• No code or Generated/ changes — diff is README.md, docs/physics-domains-guide.md, docs/complete-library-guide.md.
• Examples were sanity-checked against the actual emitted types (Velocity3D.g.cs, Force3D.g.cs, Length.g.cs, etc.) and the PhysicalConstantsGenerator output to make sure every snippet would now compile.
• No CI relevance beyond markdown rendering.

https://claude.ai/code/session_01Tj63Rddvs9frqLUgsjNEP5

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