docs: the river-plan tool in the generator README (terrain-water task 21)

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

Tools/Scripts/DrainageAnalysis.cs.uid

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Tools/Scripts/README.md

@@ -81,6 +81,17 @@ Generates the entire 2D blueprint. Roughly in order:
    steepest descent on a regular grid can only route along eight headings, and at map scale that
    reads as straight hatching, not drainage; the droplet model is the working replacement (its
    carve field measures isotropic to within 1.5 % across the folded 45° grid period).
+   **River plan (task 21, C0b part 1)** — `DrainageAnalysis.cs` + the headless
+   `RiverPlanTool.tscn` produce a river PLAN from an erosion-ON blueprint: priority-flood with a
+   one-ulp epsilon resolves the ~15k erosion pits for ROUTING ONLY (terrain untouched), deep+large
+   depressions survive as terminal basins, D8 flow directions + Kahn accumulation build the
+   drainage network, and the top ~3 TRUE-ocean outlets (the ocean body, WBID 1 — enclosed lagoons
+   do not count as "the sea") become trunk candidates with lean tributaries, mountain-exit handoff
+   points, and lean endorheic terminals. Output is a JSON *plan* sidecar + console report —
+   deliberately NOT a blueprint section, so a plan can never masquerade as realized water. Part 2
+   (task 22) carves and waters the gated plan. Pure analysis: the source blueprint is never
+   written.
+
 3. **Sea level and water** — sea level per the configured model (`SeaLevelModel`: `"flat"` scalar
    — the default, `SeaLevelValue` 0.15 — or the legacy `"field"` latitude Lerp); flood fill
    separates true ocean from inland lakes; a mainland fill guarantees one contiguous landmass.

Tools/Scripts/RiverPlanTool.cs.uid

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+uid://b37tm1tov0x6m