Translate the index entries across the French edition

[mmcky]

Emile ruled on #78 (the ruling, 2026-09-23) that the index entries should be in French throughout the edition, applying the same approach "to the remaining index entries in one pass", and #82 confirms that pass. The translation engine leaves index entries in English, so most of them still are: of the 127 entry lines in the edition's {index} blocks, the 18 in functions.md, oop_intro.md, python_by_example.md and python_oop.md are in French and the other 109 match the English. The 24 inline {index} roles are mixed.

Steps

1. Check [Experimental] Build the lectures with QuantEcon/mystmd instead of jupyter-book lecture-python-programming#363 first. That experimental English pull request, merged as it stands, would remove 119 of the English series' 128 {index} blocks and, by sync, the French entries with them. If it is close to merging into main, translate only what will survive it.
2. Translate every entry on main in one pull request: the {index} blocks and the inline {index} roles. Keep the MyST syntax (single: and pair:, and the ; between an entry and its sub-entry) and the French typography. Use the glossary's terms and Emile's rulings, with the entries already in French as the model, such as « Objets », « Identité », « Contenu » and « Méthodes » in oop_intro.md.
3. List the entries in one table on the pull request, English → French with where each occurs, so that the review is of the terms rather than of each file. Emile approves the pull request; Matt merges it, and the merge closes this issue.
4. Keep the pass from eroding. While the engine leaves index entries in English, a sync that rewrites a section can bring its entries back in English. Open an issue in QuantEcon/action-translation for French index entries in drafts and syncs (none exists yet), and check the entries in each French sync pull request until it is done.

If #82 adopts fresh drafts, each draft has its entries translated before it opens, as #82 says; this pass covers the lectures already on main.

Done when

• every {index} entry on main is in French, through one pull request that Emile has approved and Matt has merged, which closes this issue
• the action-translation issue for French index entries is open and linked here