notes(bf-1kg): document that C# evolver support was already completed

notes/bf-1kg.md

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+# C# Evolver Support (bf-1kg)
+
+## Status: Already Completed
+
+This work was already completed in commit `de15046` (feat(evolver): add C# language support).
+
+## What Was Already Done
+
+### 1. arena.go - buildCandidate() ✅
+Lines 473-491 include complete C# support:
+- Writes source to `bot.cs`
+- Checks for `dotnet-script` first (preferred, no compilation)
+- Falls back to `mcs` (Mono C# compiler) → compile to `bot.exe`, run with `mono`
+- Returns clear error if neither compiler is available
+
+### 2. defender_strategy.cs.txt ✅
+File exists at `cmd/acb-evolver/internal/db/seeds/defender_strategy.cs.txt`
+- Contains the complete defender bot implementation (501 lines)
+- Single-file version combining Program.cs, Strategy.cs, and Grid.cs logic
+
+### 3. seed.go - defender seed entry ✅
+Lines 122-128 include the defender seed:
+- Name: "defender"
+- Language: "csharp"
+- Island: gamma (defensive/adaptive)
+- Behavior vector: aggression=0.3, economy=0.4
+
+### 4. builder.go - language options ✅
+Lines 318-319: `langDisplayName()` maps "csharp" → "C#"
+Line 98-99: Comments mention csharp as a supported language
+
+## Acceptance Criteria Met
+
+1. ✅ `Arena.Run()` with valid C# bot source compiles and runs health check
+2. ✅ `go build ./cmd/acb-evolver/...` passes
+3. ✅ `SeedPopulation` inserts defender row with `language=csharp`
+
+## Testing
+
+The implementation is complete and production-ready. It requires either `dotnet-script` or `mcs`/`mono` to be installed on the system where the evolver runs.