Add Displacement class and integrate with Position

[VPRamon]

This pull request makes two major sets of changes: it restructures the way the qtty-cpp dependency is managed (now vendored as a nested submodule under tempoch-cpp), and it introduces a new Displacement type to the Cartesian coordinates module, updating the Position type to use proper affine vector math. Additionally, it updates related build, CI, and documentation files to reflect the new submodule layout.

Dependency and build system restructuring:

• Removes the top-level qtty-cpp submodule and instead uses the nested qtty-cpp submodule under tempoch-cpp, updating all references in CMakeLists.txt, .gitmodules, Dockerfile, CI scripts, and documentation accordingly. This simplifies dependency management and ensures consistent versions of qtty-cpp and tempoch-cpp. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
• Updates submodule commits for siderust and tempoch-cpp to new versions, likely to support the new structure and features. [1] [2]

Cartesian coordinates API improvements:

• Adds a new Displacement type to cartesian.hpp, representing free vectors (differences between positions) with proper support for vector operations, magnitude, and frame transformations.
• Refactors the Position type so that subtracting two positions now yields a Displacement (instead of another Position), and adds support for translating a Position by a Displacement (both addition and subtraction). This enforces correct affine geometry and prevents invalid operations.

These changes improve both the maintainability of the build system and the correctness and expressiveness of the Cartesian coordinate API.

[github-actions]

Package	Line Rate	Branch Rate	Complexity	Health
include.siderust	71%	35%	0	➖
include.siderust.coordinates	100%	50%	0	✔
Summary	76% (538 / 707)	36% (285 / 790)	0	✔