Adds concrete documentation for how +k8s:alpha(since: "1.N") and
+k8s:beta(since: "1.N") declarative validation lifecycle tags are
enforced, graduated, and owned. Touches six sections of the KEP:

- Solution: Lifecycle Tags — corrects the tag syntax (quoted
  since: string, no v prefix), documents mandatory since: in
  staging/src/k8s.io/api/**, records the community-driven
  ownership model with subproject leads as escalation path, and
  calls out the intentional format divergence from the older
  +k8s:prerelease-lifecycle-gen:introduced=1.N marker.

- Execution & Authority Logic — adds a "two stability axes"
  note distinguishing TagStabilityLevel (is the +k8s:minimum
  tag definition Alpha/Beta/Stable?) from ValidationStabilityLevel
  (is this specific application of the tag wrapped in +k8s:alpha
  or +k8s:beta?). The runtime enforcement table reads the
  validation-instance axis; the lint rules read both. This
  distinction is the main comprehension hazard contributors hit
  on first encounter.

- Linter — new "Lifecycle Tag Lint Rules" subsection
  enumerating every lint rule (existing + new in 1.36 and 1.37)
  with its enforcement home, rationale, and error behavior.
  Two rules live in kube-api-linter (alphaBetaPrefix stays where
  it is; conflictingmarkers with the
  declarative_validation_lifecycle_alpha_vs_beta conflict set
  enforces alpha ⊥ beta per field, landing as a config-only
  change in 1.36). The rest live in validation-gen's own
  internal lint harness:
    * lifecycleSinceRequired (1.37) rejects wrappers without
      since: in k8s.io/api/** packages
    * lifecycleStaleSince (1.37) warns at N+1 and errors at N+2
      past the since: value for alpha (shifted by one for beta),
      supporting per-field exemption via
      // +k8s:validation-lifecycle-exempt=<reason>
  All enforcement is generation/lint time — deliberately not
  runtime, to avoid the #127249 cautionary tale where a binary
  version bump silently changed API availability.

- Rollout Strategy Phase 2 — documents that lifecycle lint rules
  land across 1.36 (conflictingmarkers config only) and 1.37
  (lifecycleSinceRequired + lifecycleStaleSince in
  validation-gen, plus the warning pipeline that enables the
  non-blocking stale-since reminder).

- Lifecycle: Promotion Process (Continuous) — rewrites the
  three-step flow with the state machine diagram, the entry
  rules table (including the legacy fast-path carveout for
  pre-v1.36 fields migrating from handwritten validation with
  existing soak history), the soak definition and prerequisites
  for each graduation step with resource-level attribution
  accepted, per-step ownership language, and an explicit
  Non-goals block rejecting runtime enforcement, auto-graduation,
  metric-driven enforcement in-lint, and cross-version
  consistency linting.

- Example Walkthrough — fixes the (since:v1.39) typos (bare
  form with v prefix) to match the real (since: "1.39") quoted
  format that validation-gen's parser actually accepts.

- Graduation Criteria — adds a clarifying paragraph that this
  section governs tag *definition* graduation (the
  TagStabilityLevel axis for e.g. "is +k8s:minimum a Beta tag or
  a Stable tag?") and points at the per-rule Lifecycle Promotion
  Process above for validation-instance graduation. These are
  two independent axes and contributors reading the KEP for the
  first time conflate them.

The accompanying implementation lives in kubernetes/kubernetes
branch kep-5073-lifecycle-tag-linting (to be opened as a PR
separately). It adds:
- hack/kube-api-linter config for conflictingmarkers alpha ⊥ beta
- validation-gen/lint warning pipeline
- validation-gen/lint lifecycleSinceRequired rule + 9 unit tests
- validation-gen/lint lifecycleStaleSince rule (error + warning
  variants) + 21 unit tests
- validation-gen --current-release flag with fallback text-read
  of staging/src/k8s.io/component-base/version/base.go
- staging/src/k8s.io/code-generator/cmd/validation-gen/docs/lifecycle-tags.md
  reference doc for contributors
- hack/update-codegen.sh wiring to pass --current-release
  explicitly in every presubmit run