- Bump go directive from 1.25 to 1.26 in go.work and all 5 module go.mod files
- Update Dockerfile build stage from golang:1.25-alpine to golang:1.26-alpine
- Upgrade cockroachdb/swiss to fix Go 1.26 runtime compatibility
  (old version used go:linkname for fastrand64/hashFn which broke in 1.26)
- Refactor cmd/wal/perf to use sync.WaitGroup.Go() (new in Go 1.26)
  and extract functions to reduce cognitive complexity
- Fix lint issues newly visible after successful compilation:
  - Add filepath.Clean for gosec G703 path traversal taint analysis
  - Add default cases to type switches for revive enforce-switch-style
  - Preallocate slices with proper capacity for prealloc linter
- Update .golangci.yaml:
  - Exclude gosec G703 (taint-tracked config file paths)
  - Exclude gosec for maelstrom/ test harness
  - Exclude revive var-naming for stdlib-conflicting package names
  - Fix deprecated goconst ignore-strings -> ignore-string-values

Signed-off-by: Matteo Merli <mmerli@apache.org>

Signed-off-by: Matteo Merli <mmerli@apache.org>