feat(core): accept readonly vectors in vec2 operations

[RodrigoHamuy]

Adds RVec2 (Readonly<Vec2>) and uses it for every vec2 parameter that is only read from. Output parameters stay mutable Vec2.

Opening as a draft to check whether you like the approach before I extend it to the rest of the types.

Why: callers holding a readonly vector (a frozen constant, a value from a readonly struct, or anything typed as const) currently have to copy or cast it to pass it to vec2.add, vec2.length, and friends, even though those functions never write to it. Encoding that in the signature also documents which arguments are outputs.

Compatibility: Vec2 is a tuple, so Readonly<Vec2> only widens the input side. Mutable vectors stay assignable to RVec2, so existing callers are unaffected. Passing a readonly vector as out becomes a compile error, which is the intended behaviour.

Scope: vec2 only, 44 functions. create, fromValues, set, zero, and fromBuffer are unchanged since they have no read-only vector params.

Checks: tsc --noEmit clean, all 1154 tests pass. I also typechecked a temporary fixture confirming readonly inputs are accepted, mutable inputs still work, and a readonly out is rejected.

Question for maintainers: happy for me to extend this to Vec3, Vec4, Mat2/Mat2d/Mat3/Mat4, Quat/Quat2, Euler, Polar, and Spherical? Those are more involved than vec2 -- the types are consumed across shapes/, geometry/, and ik/, where internal helpers thread vectors into each other, so the readonly-ness will cascade a fair way. Also worth settling the naming (RVec2 vs ReadonlyVec2) before it's repeated a dozen times -- gl-matrix uses ReadonlyVec2, so that may be the more familiar choice.

next steps

If this seems okay, then we can do the same for all of these:

core --- tuple types that get an R* alias

0. Vec3 → RVec3
1. Vec4 → RVec4
2. Quat → RQuat
3. Quat2 → RQuat2
4. Mat2 → RMat2
5. Mat2d → RMat2d
6. Mat3 → RMat3
7. Mat4 → RMat4
8. Euler → REuler
9. Spherical → RSpherical
10. Polar → RPolar

color
11. Color → RColor
12. HSL → RHSL

shapes --- tuple types with an R* alias
13. Box2 → RBox2
14. Box3 → RBox3
15. Frustum / FrustumCorners → RFrustum / RFrustumCorners

shapes --- object types (readonly via Readonly<...> on the fields, e.g. RSphere = { readonly center: RVec3; readonly radius: number })
16. Sphere → RSphere
17. Circle → RCircle
18. Plane3 → RPlane3
19. OBB3 → ROBB3

This is all done in:

• feat: accept readonly inputs across the package #53

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