feat(sources): fetch linked URLs for the 252 spreadsheet-only cases

Until now only cases 357+ (plus a few older ones) had their linked URLs
fetched; the other 252 dossiers held just the text pasted into the
spreadsheet, often only the lede. This pass fetches every linked URL for
those cases so the full text is on file before link rot takes it.

- stage_all.py: new --include-sheet-cases flag that lifts the skip on
  cases holding spreadsheet articles. Resumable as before.
- staging/: raw extracts and index.json for the 252 cases.
- sourced-articles.json, case-files-manifest.json: 188 new UNTRIMMED
  blocks. 179 cases gained at least one fetched article; 73 got none
  (49 BLOCKED, 18 THIN, 9 DEAD, 4 video/social, 3 ERROR).
- sources/*.txt: rebuilt with build_cases.py --all. Sheet pastes are
  kept; fetched blocks sit beside them under the NOT YET TRIMMED banner.
  Files previously committed with CRLF are now LF.
- wayback-availability.json: Wayback snapshots exist for 73 of the 98
  failed URL slots; input for an archive-fetch pass.
- README-START-HERE.md: progress note and next step.

verify_all.py PASSES (1836 checks).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GZZENdTLzNGsbNy4DyF1yt

merge-output/staging/083/01_mendovoz-com.txt

@@ -0,0 +1,11 @@
+Le robaron $350 mil a un joven en Guaymallén | MendoVoz
+
+Los delincuentes sorprendieron al joven cuando este había pactado la venta de bitcoin.
+
+Este jueves, un joven de 26 años fue víctima del delito y del engaño. El hecho ocurrió cerca de las 19.30 en la calle 2 de Mayo y Severo del Castillo del departamento de Guaymallén.
+
+El robo se produjo cuando el hombre se dirigía al lugar para venderle a unas personas la moneda virtual bitcoin y en un momento unos delincuentes lo sorprendieron y le sustrajeron un iPhone, 120 mil pesos y 2500 dólares.
+
+Horas más tarde, personal policial en encontró el iPhone entre las malezas de un descampado de la calle Reconquista
+
+Intervino en el caso la Oficina Fiscal N°19 de la Comisaría 35°

merge-output/staging/083/index.json

@@ -0,0 +1,16 @@
+[
+  {
+    "case": 83,
+    "n": 1,
+    "field": "url",
+    "url": "https://www.mendovoz.com/actualidad/policiales/2021/3/19/le-robaron-350-mil-un-joven-en-guaymallen-98425.html",
+    "host": "mendovoz.com",
+    "fetched_at": "2026-09-05T12:53:56+00:00",
+    "verdict": "FETCHED",
+    "method": "raw",
+    "http": 200,
+    "chars": 730,
+    "sha256": "3d6277b9ef36208f488698136ca7812600b0659aa566b32a79f8e6332e26e414",
+    "final_url": "https://www.mendovoz.com/actualidad/policiales/2021/3/19/le-robaron-350-mil-un-joven-en-guaymallen-93818.html"
+  }
+]