chore(operator): replace go.uber.org/zap with stdlib log/slog

[lusoris]

Summary

Replaces go.uber.org/zap with the standard-library log/slog package in
cmd/vmafx-operator/ — the last of 25 Go binaries on the fork still using
the kubebuilder-template-default zap logger. All other vmafx Go binaries
(vmafx-server, vmafx-controller, vmafx-tune, MCP server, …) already
log via slog. The operator now bridges slog.NewJSONHandler into
controller-runtime via logr.FromSlogHandler, giving the fork a single
uniform logging stack across its entire Go surface.

Audit modernization item #3 (zap holdout retirement).

Type

• chore / refactor — no behaviour change, dependency cleanup.

Dependencies removed (direct)

• go.uber.org/zap → now indirect only (pulled in by controller-runtime internals; upstream's choice, unaffected).
• github.com/go-logr/zapr → dropped from go.mod entirely.
• go.uber.org/multierr → dropped from go.mod entirely.

Promoted to direct dep:

• github.com/go-logr/logr (was indirect; now used by the logr.FromSlogHandler bridge).

Bug-status hygiene (ADR-0165)

• no state delta: pure dependency-modernization refactor, no bug
  fixed/opened/ruled-out and no user-visible behaviour change.

Netflix golden-data gate (ADR-0024)

• I did not modify any assertAlmostEqual(...) score in the Netflix golden Python tests.
• No Python or C touched at all — Go-only change.

Deep-dive deliverables (ADR-0108)

• Research digest — no digest needed: mechanical 1-importer refactor with no design surface.
• Decision matrix — no alternatives: only-one-way fix (drop zap, use the slog handler all other vmafx binaries already use).
• AGENTS.md invariant note — added rule chore(deps): Update dependency anthropic to >=0.104.1 #6 to cmd/vmafx-operator/AGENTS.md documenting the slog bridge so a future kubebuilder template re-scaffold doesn't silently reintroduce zap.
• Reproducer / smoke-test command — pasted below under "Reproducer".
• CHANGELOG fragment — changelog.d/changed/zap-to-slog-uniform.md.
• Rebase note — head entry in docs/rebase-notes.md ("vmafx-operator zap → slog uniformity"). Netflix/vmaf has no operator so cross-repo impact is zero; the note covers the kubebuilder-template re-scaffold path instead.

Reproducer

# Build + vet
go build ./...
go vet ./cmd/vmafx-operator/...

# Tests (require envtest assets)
export KUBEBUILDER_ASSETS=$(setup-envtest use 1.31 -p path)
go test -count=1 ./cmd/vmafx-operator/...

# Verify zap is no longer a direct importer
grep -rn 'go.uber.org/zap' --include='*.go' .   # → no matches

Local run on master tip + this branch: go build ./... clean,
go vet clean, golangci-lint run ./cmd/vmafx-operator/... reports
0 issues., go test ./cmd/vmafx-operator/... passes
(ok 5.306s, 3 specs in the envtest suite).

Known follow-ups

None. zap remains an indirect transitive dependency of
sigs.k8s.io/controller-runtime; eliminating it entirely would require
forking controller-runtime, which is out of scope.