Add automatic exports and visualization artifacts (decay/slotting/scenarios/methods)

[junsier9]

Motivation

• Provide the four required presentation/export artifacts (decay profile, slotting table, scenario results, methods/assumptions) from a single run of python -m src.run without changing core computation logic.
• Make plot outputs compliant with presentation requirements (matplotlib, clear titles/axes/units) and ensure output file names are written to outputs/.
• Add a concise machine-readable method_summary.md suitable to paste into PPT/report and a console summary highlighting worst-case scenarios.

Description

• Added METHOD_SUMMARY_MD constant in src/config.py and a write_method_summary writer in src/irrbb/report.py that emits the required methods/assumptions text (fields/diagnostics, decay model, 5Y cap, bucket defs, shock functions with parameters, EVE/NII formulas).
• Changed build_decay_profile in src/nmd/repricing_profile.py to accept weights and balance and to output the requested schema: bucket_label, t_start_years, t_end_years, t_mid_years, weight, survival, amount.
• Reworked plotting in src/utils/plotting.py to produce: plot_decay.png (survival S(t) + bucket weight bar), plot_slotting.png (bucket_label vs amount bar), and plot_scenarios.png (grouped bars for ΔEVE and ΔNII(1Y)).
• Updated src/run.py to call the new build_decay_profile(..., balance=b0), persist decay_profile, slotting_table, scenario_results, and method_summary.md, print a console scenario summary table with worst-case markers, and validate exactly 4 shock scenarios are produced; core engines for decay fitting, slotting, EVE and NII computations were not modified.

Testing

• Compiled key modules with python -m py_compile src/run.py src/irrbb/report.py src/nmd/repricing_profile.py src/utils/plotting.py src/config.py and the compilation succeeded.
• Attempted end-to-end run with python -m src.run but it failed due to missing pandas in the execution environment.
• Attempted pip install -r requirements.txt to resolve dependencies but installation failed due to proxy/network restrictions (HTTP 403), so automated end-to-end file generation could not be verified in this environment.

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Codex Task