274-dev: drop デチン沢川; watercourse shortlist down to 31

Too thin to carry a line. Three exclusions now recorded with reasons
(名称不明, ベリベツ川, デチン沢川) so a later sweep does not reintroduce them.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S6MpcBisZYhX45oDGRRJoz

wip/274-dev.md

@@ -103,7 +103,7 @@ size OSM splits into separate ways.
 Cross-checked against the 国土数値情報 河川データセット (W05) via the Geoshape repository at
 国立情報学研究所 — all 561 named rivers of the 石狩川, 鵡川 and 沙流川 systems, since named
 rivers each carry their own 河川コード and are not in the 「その他」 group. The sweep found
-34 names; two were dropped by the author (below), leaving **32**. Of those, 31 appear in
+34 names; three were dropped by the author (below), leaving **31**. Of those, 30 appear in
 both W05 and OpenStreetMap, and all but one agree on position to within 110 m.
 
 Offset is closest approach; under ~30 m means the road crosses it.
@@ -135,7 +135,6 @@ Offset is closest approach; under ~30 m means the road crosses it.
 | 99.93 | 145 | 右の沢川 | 沙流川水系 | both |
 | 99.98 | 79 | ホロカワウシャップ川 | 沙流川水系 | both |
 | 100.66 | 21 | パンケウシャップ川 | 沙流川水系 | both |
-| 102.90 | 32 | デチン沢川 | 沙流川水系 | W05 only |
 | 103.12 | 23 | 新川 | 沙流川水系 | both |
 | 104.39 | 15 | ペンケウシャップ川 | 沙流川水系 | both |
 | 105.99 | 181 | パンケユウケシュオマナイ川 | 沙流川水系 | both |
@@ -148,9 +147,7 @@ Excluded by the author, 2026-09-10:
 - **名称不明** (63.02 km) — W05 carries it with no name; dropped.
 - **ベリベツ川** (37.38 km in OSM, never within 300 m in W05) — mostly converted to farm
   waterway, and it sits marginally west of 川端橋, outside the range.
-
-**デチン沢川**, 102.90 km, is 沙流川水系, 河川コード 8101070000 — the 「その他」 group rather
-than a named river of its own. Pending the author's check against the NII database.
+- **デチン沢川** (102.90 km, 沙流川水系, 河川コード 8101070000) — too thin to carry a line.
 
 **サヌシュッペ川** is not a disagreement: it runs beside the road rather than crossing it,
 so each dataset is tangent at a different reach (62.46 / 67.62). The crossing is at