[acamarata]

Fixes the red Coverage workflow (run 40321895084): covfloor reported internal/cmdregistry at 95.0% against a 96% floor.

Adds internal/cmdregistry/branches_test.go with behaviour assertions for the uncovered branches:

• EffectiveJSON (was 0%): no override, unset override flag, legacy/none override, none beats legacy, nil command
• bracketed / parseArgs with an unclosed bracket and nested brackets
• Build with nil root / nil canon; unresolved V15OnlyEnvelope path
• v1.4 builtin plugin mount keeps its pre-split canon classification and has no plugin identity
• validateConfirm problems: not runnable, duplicate/empty flag, plan without flag/id_flag
• warnPlugin warns once per slug; invalid plugin confirm JSON is dropped with an E437 warning
• ApplyFlagEnv with a nil registry

The floor is unchanged; no nolint or exclusions.

Commands run and results

• go test -mod=vendor -coverprofile=c.out ./internal/cmdregistry/: 85 passed, coverage 95.0% -> 99.6%
• go run -mod=vendor ./tools/covfloor -profile c.out -floors .github/coverage-floors.txt: PASS internal/cmdregistry 99.6% floor 96% (package-only profile, so not run with -strict; the full-repo profile runs in CI)
• gofmt -l and go vet -mod=vendor on the package: clean

Not covered: the encode-error branch in Registry.Marshal (needs an unencodable value).