Add mixed-mode stepping with a focus-set policy (new default)

[KristofferC]

Interpreting everything through JuliaInterpreter makes stepping over
Base/stdlib/dependency calls painfully slow, while marking them compiled
via the JuliaInterpreter globals breaks stepping into user callbacks
passed to higher-order functions (map, sort(by=...), broadcast, ...).

This adds a MixedInterpreter that decides per call, at runtime: a call
runs natively unless the callee or any argument carries a function or
type from the "focus set" - the modules being debugged. The check walks
runtime types including type parameters, so callbacks survive wrapper
chains (Generator, Broadcasted, kwcall NamedTuples, Base.Fix1, ...) and
calls dispatching on focus types stay interpreted, with no per-method
blacklist. On a sum(sort(rand(100_000))) benchmark, n drops from
218s to 0.27s (~800x).

The focus set is seeded per session: Main, packages loaded from dev
checkouts (pkgdir outside the depots' read-only packages/ stores), and
the entered method's package (except Base/stdlibs). Stepping into a
dependency frame adds it for the session with an info message; stepping
into Base/stdlibs prints a hint instead. For soundness the fast path is
suspended (with a message) while break_on(:error/:throw) is active or a
breakpoint targets a module that native code could reach, including
transitively through reverse dependencies.

UI/API:

• three modes: interpreted, mixed (default), compiled; prompt shows
  1|mixed> / 1|compiled> with per-mode colors
• M toggles mixed<->interpreted, C keeps toggling compiled and
  returns to the previous mode; new mode [interpreted|mixed|compiled]
• new focus command: list the set, focus add Mod, focus rm Mod|i
• Debugger.set_default_mode!/default_mode, interpret_module!/
  compile_module!/reset_module!, interpreted_modules()

Documented limitations (escape hatch: interpreted mode): type-erased
callbacks (Vector{Any}, ::Function fields), type piracy, and ambient
execution (registries, Tasks, finalizers).

Requires JuliaInterpreter 0.11 (native_call runs in frame.world, giving
consistent Revise semantics).

Co-Authored-By: Claude Fable 5 noreply@anthropic.com