docs: verify legacy migration paths on published packages

[stefan-jansen]

Extend the existing Pyfolio/Alphalens guide and add one selected Empyrical metric guide with a runnable shared-input comparison. Ten complete-input portfolio metrics are covered; annualization, risk-free units, NaN VaR, alpha/stability differences, unsupported grouped/group-neutral behavior and full-tearsheet limits are explicit. Sharpe and beta are dimensionless. The guide command uses --task all explicitly; the script's base-dependency default remains runnable.

Current head df7c7ca is based on main dc9768c. Stable patch ID cd13e87835697d12e9d57bb0bf9c5533b16d0ee4 proves the focused docs/example diff is unchanged from original tested head 942f367.

Fresh checks at this head:

• The documented published four-package command with --task all passed on Python 3.12: Diagnostic 0.1.8, Pyfolio 0.9.9, Alphalens 0.4.6, Empyrical 0.5.12; NumPy 2.5.4, pandas 2.3.3, SciPy 1.18.1, Polars 2.0.0.
• The separate published Empyrical command passed with pandas 3.0.6 and the documented pytz 2026.5 workaround. Observed numerical agreements and differences match the guide; no new parity claim is made.
• Full locked source suite: 5484 passed, 7 skipped on Linux/Python 3.14.3. Ruff check/format, ty and all-file pre-commit passed.
• Strict MkDocs, 9665 README/rendered links and 20 external destinations, and library/version/commit identity passed. The old intermittent external 503 is historical, not a current blocker; the bounded checker repair is merged as fix(docs): retry transient link failures with bounded checks #105.
• Wheel/sdist contents and metadata, installed-wheel imports and README, and all 11 principal task-doc examples passed.
• Required current-base hosted CI remains mandatory before merge.

Working revision-pinned guides (actual GET 200):

• https://github.com/ml4t/diagnostic/blob/df7c7ca5aa8b8f097875289399d20e92dc0d1118/docs/user-guide/migration.md
• https://github.com/ml4t/diagnostic/blob/df7c7ca5aa8b8f097875289399d20e92dc0d1118/docs/user-guide/empyrical-migration.md

The guides link existing purged-validation and Backtest reporting workflows as supported reasons to evaluate Diagnostic. No speed or full API-replacement claim is made. Merging updates source documentation only; canonical website deployment and package publication are separate user decisions.

AI assistance: Codex prepared the docs/example, reproduced the public workflows, reviewed the focused diff and ran the checks. Maintainer accountability remains required.

Closes #94.

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