fix: disambiguate duplicate Verso labels in Section 6.2

[Chessing234]

Section 6.2 has three groups of declarations sharing one docstring:

• Definition 6.2.3 (Ordering of extended reals) — 2 (the ≤ and < characterisations)
• Examples 6.2.4 — 4 examples
• Definition 6.2.6 — 5 theorems covering the different supremum cases

Same convention as #624: the definitions name the case they cover ((supremum of the empty set), (supremum ignores negative infinity), …), the examples take part letters as in #634. Docstrings only.

Checked: no duplicate one-line docstrings remain, all edited lines within 100 characters.

lake build Analysis.Section_6_2 succeeds locally — Build completed successfully (3273 jobs). The one info: line it prints is a pre-existing #check-style trace at Section_6_2.lean:177, unrelated to this change.

[teorth]

Thanks — the Definition 6.2.3 and Definition 6.2.6 disambiguators are exactly the right style, and I've merged the sibling PRs that use it throughout.

One thing to adjust first. Examples 6.2.4 is a single prose sentence in ch6, not an enumerated list:

$3 \leq 5$, $3 < +\infty$, and $-\infty < +\infty$, but $3 \not\leq -\infty$.

There are no parts (a)–(d) in the text, so labelling the four Lean examples that way asserts structure the book doesn't have. Could you switch to descriptive parentheticals, matching the style you used for Definition 6.2.3 in the same PR? For instance (two reals), (a real and positive infinity), (negative and positive infinity), (a real is not below negative infinity).

The rest is good to merge.

[Chessing234]

Done in 37af0f0 — Examples 6.2.4 now reads (two reals), (a real and positive infinity), (negative and positive infinity), (a real is not below negative infinity), matching the order of the four examples. The Definition 6.2.3/6.2.6 labels are unchanged.