Add user-defined invoice set structure dialog (ADR 0016)

Lets a back-office user correct/define which invoice line belongs to
which "set" and how deep it is nested, instead of relying only on the
implicit "everything after a Set header until the next Set" grouping
fds__prepInvoice derives. Sets may contain sets; every change applies
immediately to the cached draft session and is reflected in the editor
and PDF (ADR 0006/0008).

Backend:
- InvoiceSetStructure: pure parent-link forest (assign, change level,
  create, place/move-sibling, depth-first linearise + level recompute).
- InvoiceDraftSetStructureDeltas: new inv/dpatch targets set.assign,
  set.level, set.create, set.move.
- InvoiceSetPricing.Build is now order-preserving and nesting-aware
  (governed by the nearest converted ancestor); the set-item switch
  sums a header's transitive descendants so nested conversions never
  double-count.
- FuchsPdf indents nested set members per level, mirroring the editor.

Frontend:
- New fis.inv_sets.js: "Set-Struktur bearbeiten" dialog — per-block
  table of all lines with drag & drop reorder (before/after/into),
  ▲▼ order and ◀▶ level buttons, fold/unfold per set (view-only), and
  a create bar for new Set/Text lines. Posts one delta per action; the
  dialog is a pure view of the server state (ADR 0006).
- _Layout.cshtml / fis_main.js: cache-bust the dynamically imported
  per-module bundles (web/fis.<mdl>.<locale>.js/.css) with an asset
  version derived from the newest bundle write time, so a gulp rebuild
  is picked up without a stale cached module.

Docs: ADR 0016, live-draft-editing concept, INVOICE_SET_PRICING.md,
USER_GUIDE.md, ARCHITECTURE.md updated. Tests: InvoiceSetStructureTests
(new) + additions to InvoiceDraftServiceTests/InvoiceSetPricingTests.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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

@@ -1,9 +1,10 @@
 ---
 status: Active
-lastUpdated: 2026-07-10
+lastUpdated: 2026-09-09
 applyTo:
   - "Fuchs/Services/InvoiceDraft*"
   - "Fuchs/Services/IInvoiceDraft*"
+  - "Fuchs/code/InvoiceSetStructure.cs"
   - "Fuchs/Services/ReminderDraft*"
   - "Fuchs/Services/IReminderDraft*"
   - "Fuchs/code/InvoiceDraftSession.cs"
@@ -18,6 +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"
 ---
 
 # Live draft editing (backend-authoritative invoice previews)
@@ -83,6 +86,19 @@ Expiry:   Server (timer) --SignalR draftExpiring{token,secondsLeft}--> warn "bit
   `BuildInvoiceRequestList`), applies deltas (`ApplyDelta`), builds the view-state DTO,
   flushes to the DB by reusing `IInvoiceService.RegisterInvoiceAsync` (no new persistence
   path), renders previews from a synthesised registration, and discards by reloading.
+- **Set structure** (ADR 0016). Every cached line carries a parent link (`SetItmId` → the
+  owning `typ == "set"` line, null/self = top level); sets may nest. `fds__prepInvoice`'s
+  implicit tagging is the initial structure; the user edits it in the "Set-Struktur
+  bearbeiten" dialog (`$inv.sets`, `fis.inv_sets.js`) which posts one delta per action —
+  `set.assign` { parent } (drop onto a set / pull out), `set.level` ±1 (indent under the
+  preceding sibling set / outdent to the grandparent), `set.create` { typ: set|text, title,
+  parent }, `set.move` ±1 | { anchor, where } (reorder among siblings / drag before or after
+  a row; the subtree travels along). `InvoiceDraftSetStructureDeltas` delegates to the pure
+  `InvoiceSetStructure` (assign / change level / create / place, then **linearise** the block depth-first and mirror
+  `setId`/`level` onto the `items` contract). `Refresh` publishes `lvl` on every line
+  (`RecomputeLevels`), rendered as an indent class in the editor and a per-level indent in the
+  PDF. `item.setprice` sums a header's **transitive** descendants. Structure deltas never
+  change values — only order, membership, levels and therefore position numbers.
 - **Cache** (`InvoiceDraftCache`, singleton) stores sessions by token with an idle sliding
   TTL; `InvoiceDraftExpiryService` (a `BackgroundService`) warns before, and evicts after,
   the TTL. TTL/warn-lead are configurable under `Fuchs:DraftEditing`.
@@ -97,12 +113,15 @@ Expiry:   Server (timer) --SignalR draftExpiring{token,secondsLeft}--> warn "bit
 ## Key files
 - `Fuchs/code/InvoiceDraftSession.cs` — session + `ChangeHistoryEntry` + `InvoiceDraftSums`.
 - `Fuchs/code/InvoiceDraftCalculator.cs` — pure recompute + validate.
+- `Fuchs/code/InvoiceSetStructure.cs` — pure set forest: parent links, levels, linearise, assign/level/create (ADR 0016).
+- `Fuchs/Services/InvoiceDraftSetStructureDeltas.cs` — the `set.*` patch targets.
 - `Fuchs/Services/InvoiceDraftCache.cs` / `IInvoiceDraftCache.cs` — in-memory store + TTL.
 - `Fuchs/Services/InvoiceDraftEditService.cs` / `IInvoiceDraftService.cs` — orchestration + delta contract.
 - `Fuchs/Services/InvoiceDraftExpiryService.cs` — idle warn/evict monitor.
 - `Fuchs/Notifications/DraftPreviewHub.cs`, `DraftNotifier.cs`, `IDraftNotifier.cs` — targeted signals.
 - `Fuchs/Controllers/IntranetController.InvoiceDraft.cs` — `inv/d*` endpoints.
 - `Fuchs/js/intranet/fis_main.js`, `Fuchs/js/intranet/modules/fis.inv_shared.js` — client.
+- `Fuchs/js/intranet/modules/fis.inv_sets.js` — the set-structure dialog (`$inv.sets`), a view of `dstate.req`.
 
 ## Reminders (Zahlungserinnerung)
 
@@ -151,3 +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)