Key the set-price menu modes on the set instead of the service-request block (ADR 0019)

"Set mit Preis" / "Nur Set mit Preis" collapsed every block into an inserted
"Gesamtumfang pauschal" row, nulled/removed all lines and cleared the user-defined
set structure. Both modes now convert every still-unconverted top-level set:
the set line takes the sum of its subtree, its members render without a price
(setprice) or are removed from itm + items (setonly); lines outside a set stay
individually priced and several sets stay separate lines. Nested sets are priced
once via their top-level set. One conversion primitive (ConvertSetHeader) now
backs all three triggers; item.setprice is a no-op on an already-converted set
(a repeated delta used to reset the set price to 0). The header's price_net is
mirrored too (flat PDF path unit price), and Build treats a null total_net as
"no price" like the flat path. The editor checks for a set before offering the
modes and the menu entry stays available for sets added later.

ADR 0009 is superseded by 0019; INVOICE_SET_PRICING.md rewritten accordingly.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

Fuchs/Docs/Concepts/live-draft-editing.md

@@ -19,8 +19,8 @@ applyTo:
 relatedDecisions:
   - "0006-backend-authoritative-draft-editing.md"
   - "0007-targeted-draft-signalr-groups.md"
-  - "0009-block-setprice-modes-are-irreversible-mutations.md"
   - "0016-user-defined-set-structure.md"
+  - "0019-set-price-modes-are-keyed-on-the-set.md"
 ---
 
 # Live draft editing (backend-authoritative invoice previews)
@@ -170,5 +170,5 @@ amount pair.
 ## Related decisions
 - [0006 — Backend-authoritative draft editing](../Decisions/0006-backend-authoritative-draft-editing.md)
 - [0007 — Targeted draft SignalR groups](../Decisions/0007-targeted-draft-signalr-groups.md)
-- [0009 — Block set-price modes are irreversible mutations](../Decisions/0009-block-setprice-modes-are-irreversible-mutations.md)
 - [0016 — User-defined set structure](../Decisions/0016-user-defined-set-structure.md)
+- [0019 — Set-price modes are keyed on the set](../Decisions/0019-set-price-modes-are-keyed-on-the-set.md) (supersedes 0009)