fix: remove Buildarch from globals, main, Makefile, logger, and tests

Buildarch was erroneously included. Remove it from:
- internal/globals/globals.go (struct field, package var, setter)
- cmd/dnswatcher/main.go (var declaration, setter call)
- Makefile (BUILDARCH var, ldflags)
- internal/logger/logger.go (Identify log field)
- internal/config/config_test.go (test fixture)
- README.md (build command, architecture section)

README.md

@@ -171,7 +171,7 @@ cmd/dnswatcher/main.go         Entry point (uber/fx bootstrap)
 
 internal/
   config/config.go             Viper-based configuration
-  globals/globals.go           Build-time variables (version, arch)
+  globals/globals.go           Build-time variables (version)
   logger/logger.go             slog structured logging (TTY detection)
   healthcheck/healthcheck.go   Health check service
   middleware/middleware.go      HTTP middleware (logging, CORS, metrics auth)
@@ -363,11 +363,10 @@ make clean      # Remove build artifacts
 
 ### Build-Time Variables
 
-Version and architecture are injected via `-ldflags`:
+Version is injected via `-ldflags`:
 
 ```sh
-go build -ldflags "-X main.Version=$(git describe --tags --always) \
-  -X main.Buildarch=$(go env GOARCH)" ./cmd/dnswatcher
+go build -ldflags "-X main.Version=$(git describe --tags --always)" ./cmd/dnswatcher
 ```
 
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