Smoother abstract / Protocol -> implementation binding, with clearer scope- and cycle-violation diagnostics

[lesnik512]

Migrated from ROADMAP.md's ## Under consideration section, which is being deleted — an unscheduled item belongs in the tracker, not in a third hand-maintained file. See #438.

Why it is open

Two related rough edges, stated as one roadmap line:

• Binding an abstract base or Protocol to a concrete implementation. Today
  this is Alias, or a Factory registered under the abstract type. Whether
  that is ergonomic enough is the open question.
• Diagnostics for scope- and cycle-violations on those graphs.

What already exists

validate() walks the graph for missing providers, scope-direction violations,
and cycles, and reports them all at once; the error classes in
modern_di/exceptions.py render the arrow-tree chain and "did you mean"
suggestions. A proposal here has to name what those messages fail to say, not
restate that diagnostics matter.

Note that 0025-alias-binds-nothing
already settles what Alias is and is not; re-proposing it as a binding
mechanism is closed.

Revisit trigger

A concrete graph whose scope or cycle error message left the reporter unable to
find the offending edge, or a binding case the current primitives cannot state.