Significantly update explanations

EXPLANATIONS.md

@@ -176,6 +176,43 @@ band-plan lookup), say that plainly and cite the band plan in
 `source` — confidence stays low (3–4) until someone finds a deeper
 hook.
 
+### The Hilfsmittel note
+
+Candidates may use the official exam aid (`Hilfsmittel_12062024.pdf`)
+during the exam. Its complete contents — every formula and table, with
+**printed page numbers** — are catalogued in
+[`references/Hilfsmittel.md`](references/Hilfsmittel.md). When a
+question's answer is a lookup or a direct application of something in
+that sheet, append a short note to the end of the `explanation` body:
+
+```
+... <u>Hilfsmittel:</u> <pointer>
+```
+
+Rules — this is the part that was historically done badly (generic
+boilerplate stamped on everything), so be strict:
+
+- **Only cite what is actually in the sheet.** Verify against
+  `references/Hilfsmittel.md`. If the fact the question turns on is a
+  memory item — diode forward voltages (~0.6 V / ~0.3 V), `tan δ = 1/Q`,
+  S-meter step = 6 dB, harmonics = n × fundamental, ppm/percent
+  arithmetic, semiconductor behaviour, definitions, antenna
+  length/shortening factors — **do not add a Hilfsmittel note at all.**
+- **Name the specific formula(s) or table, and the page.** Not "the
+  formula is in the Formelsammlung" but e.g. `P = U²/R (Leistung,
+  S.12)` or `the Widerstands-Farbcode table (S.11)`.
+- **For multi-step calculations, give the order:** "first
+  `U_eff = Û/√2` (Wechselspannung, S.12), then `P = U_eff²/R`
+  (Leistung, S.12)". State plainly when a value comes from outside the
+  sheet (e.g. a diode drop) versus from a sheet formula.
+- **For a pure table lookup**, say it is a lookup and cite the table +
+  page (e.g. "a table lookup, not a memory item — the band limits …
+  are in the Frequenzbereichszuweisung (Anlage 1, Tabellarische
+  Übersicht, S. 2–3)").
+- **Page numbering:** the printed page number = PDF page − 2 (the cover
+  and the "Hinweis" page are unnumbered). Always cite the printed
+  number, as `references/Hilfsmittel.md` does.
+
 ---
 
 ## 5. Confidence scale