feat: initial Münster Haushalt icicle viewer

Editorial single-page viewer for the City of Münster's 2026/2027
budget draft, built as an Astro v6 SPA with a 4-level zoomable
icicle (Produktbereich → Produktgruppe → Category → Breakdown).

Highlights:
- Multi-flow data layer over the official open-data CSVs
  (Aufwendungen + Erträge, 2008–2028) with overlap reconciliation
  across plan years.
- Year slider as a 21-year mini-histogram of both flows;
  drag-to-scrub and click-to-jump, with bars morphing via CSS
  transitions on SVG geometry attributes.
- Vertically centred icicle with year-outline rectangles framing
  each year's relative budget size, à la Bostock's animated treemap.
- Headline "ausgibt / einnimmt" toggle; sidebar Aufwendungen/Erträge
  rows double as flow toggles. Active flow in Aufwendungen-purple /
  Erträge-orange (OKLCH).
- Click-to-zoom via path-keyed lookup with ZOOM_COL_BOUNDS that
  reallocate the depth axis per zoom state. Zoomed item moves to the
  sidebar; canvas shows its descendants only (no adjacent-block leaks).
- Sidebar shows path-specific Aufwendungen/Erträge/Saldo plus the
  source-PDF Beschreibung; Erläuterungen behind a collapsed details.
- Build-time PDF extraction (scripts/extract-pg-sections.mjs) parses
  68 Produktgruppen' Beschreibung + Erläuterungen sections from
  Band 1, including 10 cells of structured Mio.-€ breakdowns
  (Steuern, Transferaufwendungen, etc.) that drive the level-4 view.
- URL state sync for path, year, and flow via history.replaceState
  so any zoom is shareable.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

CLAUDE.md

@@ -0,0 +1,76 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project intent
+
+Build an interactive treemap-based viewer of the City of Münster budget so non-experts can navigate where money comes from and where it goes. The 2026/2027 draft (`Haushaltsplanentwurf`) is the primary target; prior years and historical actuals are included for trend/context views.
+
+No application code exists yet — this repo currently holds only source data.
+
+## Data layout
+
+```
+data/
+  2023/                       Haushaltsplan 2023            (xlsx + csv)
+  2024/                       Haushaltsplan 2024-2027       (xlsx + csv)
+  2025/                       Haushaltsplan 2025-2028       (xlsx + csv)  ← latest machine-readable plan
+  jahresabschluesse/          Jahresabschlüsse 2008-2022    (xlsx + csv)  historical actuals
+docs/sources/
+  2024/  2025/  2026_2027/    Original PDFs (Bände 1+2, Satzung, Reden)
+```
+
+Source: opendata.stadt-muenster.de (CSV/XLSX) and stadt-muenster.de (PDFs). License: Datenlizenz Deutschland Namensnennung 2.0.
+
+The 2026/2027 draft is **not yet on the open-data portal** — only PDF. The 2025 file does include 2026/2027/2028 *planning* values, which is the closest machine-readable forward projection until the new draft is published as data.
+
+## Data schema (the part you must read multiple files to understand)
+
+CSV files share one shape across all years:
+
+- **Separator:** `;` — **encoding:** UTF-8 with BOM
+- **Number format:** German — `.` thousands, `,` decimal (e.g. `-733.670.000,00`)
+- **Sign convention:** **revenues are negative, expenses are positive** (NRW kameral/doppik convention). Normalize/flip before display.
+- **Hierarchy keys:** `Produktbereich` → `Produktgruppe`. Both can take the literal value `Gesamt` to denote a roll-up row — those rows are subtotals/totals, not leaves. Filter or use them deliberately; never sum them alongside leaves.
+- **Time key:** `Geschäftsjahr` — each plan file contains multiple years (current + 3 planning years), so a single CSV is long-format across years.
+- **Categories (columns):** ~19 fixed financial line items. Revenue side includes `Steuern und ähnliche Abgaben`, `Zuwendungen und allgemeine Umlagen`, `Öffentlich-rechtliche Leistungsentgelte`, etc. Expense side includes `Personalaufwendungen`, `Aufwendungen für Sach- und Dienstleistungen`, `Transferaufwendungen`, `Bilanzielle Abschreibungen`, etc. The plan files have one extra column `Globaler Minderaufwand` that the Jahresabschluss file doesn't.
+- **Header typo to preserve:** `Öffentlich-rechtliche Leistunngsentgelte` (double-n) appears in the source — don't "fix" it on read; map it.
+- **Spelling drift across files:** 2025 plan uses `algemeine Umlagen` (single-l), Jahresabschluss uses `allgemeine`. Treat as the same field.
+
+The two-level `Produktbereich/Produktgruppe` hierarchy is the natural treemap nesting; the ~19 category columns are switchable views (revenue vs expense, or a single category drilled across products).
+
+## Design Context
+
+(Mirror of `.impeccable.md`. The full design brief lives in `docs/design-brief.md`.)
+
+### Users
+Engaged Münster citizens and journalists. German-speaking, intelligent, time-pressured. Mixed familiarity with budget docs. Many will share findings or cite specific numbers, so deep-linking and quoteable figures matter. Treat them like adults — no pictograms, no hand-holding tone.
+
+### Brand Personality
+**Editorial, generous, opinionated.** A piece of civic data journalism, not a municipal portal and not a dashboard. Voice: composed, curious, never cute. Numbers presented with precision and context, never as "look at this big number" hero stats.
+
+### Aesthetic Direction
+**Print-rooted editorial.** Strong serif/slab display type, asymmetric grid, generous whitespace, off-white "paper" background tinted slightly warm. The treemap is the protagonist; everything else frames it like a magazine spread frames its lead photograph. Lean toward NYT/SZ/ZEIT graphics-desk references, the Pudding's longer pieces, Vignelli-grade typographic confidence.
+
+**Anti-references** (do not look like): generic SaaS dashboards, German municipal portals, AI-slop hero pages (gradient text, glassmorphism, purple-blue gradients), PowerPoint infographics.
+
+### Theme & Language
+Light only, paper-feeling. German only.
+
+### Design Principles
+1. **The treemap is the page.** Layout, type, color, and motion all serve reading it.
+2. **Print logic first.** Asymmetric, typographic, quiet. Sunday-paper-supplement-grade.
+3. **Numbers in context, never alone.** Every figure paired with its share, change, or what it pays for.
+4. **Data does the talking.** Where copy exists, it points at data — never competes with it.
+5. **Restraint is the accent.** One display face, one body face, one accent system, one motion vocabulary.
+
+### Color decided
+Two-hue system: *Aufwendungen* and *Erträge* each get a hue family in OKLCH on the warm-paper background. Tile lightness/chroma encodes value within a flow. No red/green moralizing. Specific hues to be auditioned during craft.
+
+### Stack decided
+Astro + D3 islands. Static-first. Deep-linkable URLs. SSR for share targets.
+
+## When extending
+
+- Re-downloading data: see the URLs in conversation history or fetch the dataset pages on opendata.stadt-muenster.de (`/dataset/haushaltsplan-YYYY-der-stadt-münster`). Watch for the 2026/2027 plan to appear there — that should replace the 2025 file as the primary source.
+- PDFs in `docs/sources/` are the authoritative narrative; the CSVs are the data. Don't try to OCR the PDFs while structured data exists.