* inclusion emulator logic for asynchronous backing (#4790)

* initial stab at candidate_context

* fmt

* docs & more TODOs

* some cleanups

* reframe as inclusion_emulator

* documentations yes

* update types

* add constraint modifications

* watermark

* produce modifications

* v2 primitives: re-export all v1 for consistency

* vstaging primitives

* emulator constraints: handle code upgrades

* produce outbound HRMP modifications

* stack.

* method for applying modifications

* method just for sanity-checking modifications

* fragments produce modifications, not prospectives

* make linear

* add some TODOs

* remove stacking; handle code upgrades

* take `fragment` private

* reintroduce stacking.

* fragment constructor

* add TODO

* allow validating fragments against future constraints

* docs

* relay-parent number and min code size checks

* check code upgrade restriction

* check max hrmp per cand...