docs(conferences): add tuples to lecture 4

[apiad]

Hotfix: lecture 4 is today (2026-10-07), so this skips the issue.

The 2026 plan had no place for tuples. Lecture 4 already used for i, nota in enumerate(notas) without explaining it, lecture 5 uses enumerate again, and lecture 6 (dictionaries) will need .items().

Changes:

• New section 6, "Tuplas", between the grade project and lists of lists: an immutable sequence, why it exists (a few values where each position means something), unpacking, the ValueError on a count mismatch, enumerate explained, the comma builds the tuple and (5,), return a, b, and a, b = b, a.
• A forward reference from the enumerate paragraph in section 2.
• Sections 7 to 9 renumbered, and the one "sección 6" reference in the exercises updated.
• Two summary bullets, and a new exercise 2 (minimo_y_maximo in one pass, without min/max). The old exercises 2 to 8 are now 3 to 9; the swap paragraph points at exercise 3 (invertir_en_el_lugar).
• plan.md mentions tuples in the lecture 4 entry.

The tuples section sits after the project on purpose: it uses min and max, which the project section introduces only after writing maximo by hand.

Verified: scriptorium render notas.md exits 0, the PDF went from 15 to 18 pages, and every new block's output in the PDF (the TypeError, the ValueError, [(0, 4), (1, 5), (2, 3)], 2 1) is what Python 3.14 printed. The sieve growth figure still renders.

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