Dependencies module (FS/SS/FF/SF)

[bart-krakowski]

Parent

#25

What to build

Extract the FS/SS/FF/SF event dependency system into a proper module. When registered, the module adds dependency-aware transformation to the write pipeline: moving a predecessor event automatically cascades the required shift to all dependents in topological order.

This module is the reason the Timeline/Gantt view exists as a product. Extracting it from `CalendarCore` into a module also fixes the current architecture where Gantt semantics are baked into the Calendar kernel.

Acceptance criteria

• `DependenciesModule` plugs into the kernel via the module API (Kernel + module registration API #28)
• `createDependency(sourceId, targetId, type)` creates a link and validates it does not form a cycle
• Moving a predecessor cascades the correct shift to all dependents (FS: finish-to-start, SS: start-to-start, FF: finish-to-finish, SF: start-to-finish)
• Cascade propagates transitively through the dependency graph in topological order
• `validateEventDependencies()` checks whether placing an event satisfies its predecessor constraints
• Circular dependency detection rejects the link with a descriptive error before it is committed
• Removing the `DependenciesModule` causes dependency code to tree-shake out of the bundle

Blocked by

#28