docs(design): add debug-profiling.el module brainstorm output

Captures the agreed v1 shape for a new =debug-profiling.el= module: targeted slow-command investigation, two features ("profile next command" and "time region or sexp"), each split into pure helper plus interactive wrapper. Migrates the existing =profiler-*= bindings and =cj/benchmark-this-method= out of =config-utilities.el=. Stays on the existing =C-c d= debug umbrella prefix.

Six approaches were considered: three conventional, plus three tail samples (macro-first, log-and-grep, treesit picker). Recommendation is the boring named-operation surface backed by a thin wrapper over the built-in =profiler.el= and =benchmark.el=. The other five options are recorded with reasons-rejected so a future reader can see what was weighed.

Design covers architecture, data flow, error handling, testing approach, and observability. Two open questions are parked: default REPS for =cj/time--expr=, and whether to capture =cpu+mem= or just =cpu=. Both are fine to defer until v1 has been used on the queued org-capture target-building investigation.

Implementation will run via =/start-work= against this design.

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