core: `Path` typing isn't consistent

Path is an interface that can be used to type Length, Offset, and such.

But we have significant issues with what it actually means. When we handle several types of paths, it generally means "0 = start of the first block included in the path". But there are cases where 0 means the departure point (as in, the travelled path).

That's a significant issue. We should unify this, make the semantics consistent, and document it. We should probably remove the ambiguous Path at least temporarily, and introduce a BlockPath type instead.

Might be addressed at some point (no sub-issue created, TDB if we must track it):

• type offsets in envelope framework, as noted https://github.com/OpenRailAssociation/osrd/pull/11210#discussion_r2007244553
• Rename some types for consistency about use of "sequence"/"path"/"..." https://github.com/OpenRailAssociation/osrd/issues/11259

Start with core/kt-osrd-sim-infra/src/main/kotlin/fr/sncf/osrd/sim_infra/api/PathProperties.kt and trace how Path, Length, and Offset are used for different path meanings. Review the envelope framework offset discussion and the related naming issue. Done means the path semantics are unified, ambiguous Path usage is addressed as appropriate, and the resulting types are documented.