[EPAC-1006]: Bill page — PBO costing panel

[riddim-developer-bot]

Summary

Adds the Independent costing — Parliamentary Budget Officer panel to the canonical Bill page (BillDetailView). When the backend has a PBO costing note linked to a bill, the panel shows the verbatim headline figure (5-year cost in millions), a methodology category badge, and the publication date, with a link out to the full PBO report PDF. Tapping the panel opens an in-app reader showing the PBO summary card and a button to open the PDF in Safari. When no PBO link exists, the panel renders nothing — no fallback prose.

Implements LoadPBOCosting (Clean Architecture Shape from EPAC-1006). iOS-only change; the backend GET /pbo/by-bill/{legisinfo_id} endpoint is a shipped prerequisite. The old on-device PBO scraper (PBOService/PBOReport) is removed — all PBO parsing lives in the backend; iOS decodes typed JSON only and never scrapes PBO pages on device.

Linear: https://linear.app/riddimsoftware/issue/EPAC-1006/bill-page-pbo-costing-panel

What changed

• Domain — PBOCosting entity (headlineFigureMillions, methodologyCategory, publishedAt, reportURL, sourceURL, summaryText) + PBOCostingResult (latest note + older linked notes). PBOCostingQueryPort.
• Application — LoadPBOCosting: returns nil when absent/empty; otherwise sorts notes by publication date (latest wins) and keeps older notes for the reader.
• Data — BackendPBOCostingRepository (PBOCostingQueryPort): GET /pbo/by-bill/{legisinfo_id}; 204/404 → nil (hide panel); decodes the typed JSON contract.
• Views — PBOCostingPanel (renders nothing when result == nil) + PBOCostingReaderView (in-app summary card, other-notes list, open-PDF-in-Safari). Wired into BillDetailView via injected LoadPBOCosting + autoloadPBOCosting.
• Strings — en + fr pbo.costing.* / pbo.methodology.* / common.done. Old pbo.* cost-analysis keys removed.
• Docs — LoadPBOCosting use case, PBOCostingQueryPort port, and corrected PBOCosting entity added to docs/architecture/use-case-catalog.md.
• Removed — on-device ios/epac/Util/PBOService.swift and ios/epac/Model/PBOReport.swift.

Backend JSON contract (boundary)

GET /pbo/by-bill/{legisinfo_id} → 200:

{
  "costings": [
    {
      "id": "PBO-2026-014",
      "title": "Cost Estimate — Bill C-8",
      "headline_figure_millions": "1,240",
      "methodology_category": "legislative-cost",
      "published_at": "2026-05-20",
      "report_url": "https://www.pbo-dpb.ca/en/publications/RP-2627-014-S",
      "source_url": "https://www.pbo-dpb.ca/en/publications/RP-2627-014-S",
      "summary_text": "The PBO estimates the five-year cost at $1,240 million."
    }
  ]
}

204/404 → no PBO link; the bill page hides the panel. The decoder also accepts a bare top-level array and alternate envelope keys so a backend wrapper rename does not silently blank the bill page; the canonical costings key is always tried first. Figures and summary text are rendered verbatim — no paraphrasing, no LLM summaries.

Acceptance criteria

• New "Independent costing — Parliamentary Budget Officer" panel on the canonical Bill page
• Renders headline figure (5-year cost, in millions, verbatim) when present
• Renders methodology category badge
• Shows publication date and "View full report" link to the PBO PDF
• Renders nothing when no PBO link exists — no fallback prose
• Tapping the panel opens an in-app reader for the summary card + "Open PDF in Safari"
• Snapshot tests: present, absent, multiple notes (latest wins, others linked) — plus no-figure and reader variants
• LoadPBOCosting added to the use-case catalog (DoD)

Verification

All run locally on the rebased branch (iPhone 17 Pro simulator, iOS 26.5):

• xcodebuild build + test: app and test target compile clean; 5 snapshot tests (present, presentNoFigure, multipleNotes, absent, reader) pass with 0 failures; 13 unit tests pass (LoadPBOCostingTests 7, PBOCostingRepositoryTests 6). CI's iOS job is build-only (ENABLE_TESTABILITY=NO), so the tests are run locally; CI re-runs the build as the pr-build gate.
• SwiftLint (project config, as CI runs it): exit 0, no violations in changed files. Fixed a real cyclomatic_complexity error (and a latent type-inference bug) in the inherited JSON decoder — simplified it to the documented contract.
• Localization audit (scripts/localization/check_localizations.py): exit 0; en/fr PBO keys at parity (verified French translations, e.g. "Directeur parlementaire du budget").

Simulator-rendered evidence

The panel below is rendered by the production SwiftUI view in the simulator via the snapshot harness. The bill page only shows the panel when staging returns a PBO link for a specific bill (data-dependent), so these deterministic renders are the visual evidence:

• Present — section title, bill title, "5-year cost, millions / 1,240", "Legislative cost" badge, "May 20, 2026", "View full report".
• Multiple notes — latest note in the panel with a "2 other PBO note(s)" indicator.
• Reader — summary card (with verbatim PBO summary text) + "Open PDF in Safari", and an "Other PBO notes" list.
• Absent — no PBO section renders between the sentinel "Key facts" and "Committee study" sections.

Release-Note: Bill pages now show the Parliamentary Budget Officer's independent costing — the 5-year cost, methodology, publication date, and a link to the full PBO report.