Rename Function to Assignment and dual-write K8s annotations

[scott-rc]

Function is FaaS vocabulary that mismatches Skipper's behavior. Skipper does not invoke code -- it routes traffic to tenant-supplied pods. The misnomer is most visible at x-skipper-function, the header tenants set on every request, and it leaks into the type vocabulary that contributors learn first. Assignment describes what the carrier actually represents: the binding of a pod to a tenant for a request.

The rename also clears the way for a follow-up redesign that adds a per-tenant policy surface (heartbeat timeout, retry policy, HPA tolerance, assign timeout, token TTL, transport, zone awareness) on the same carrier -- those are stuck as cluster-wide flags today, with no natural home on Function. This PR does not touch the policy surface; it sets up the vocabulary the next round of work builds on.

This PR is the internal rename plus K8s annotation dual-support. External surfaces (HTTP header, Prometheus labels, OTLP attrs, slog keys, CLI flags, web URL paths, user-visible UI labels) deliberately stay on legacy function- form here -- follow-up PRs introduce dual-support for each so tenant SDKs, dashboards, and operator manifests can migrate at their own pace.

The one external surface this PR changes is K8s pod annotations: the controller now dual-writes both skipper/function and skipper/assignment on every assigned pod, and the read path prefers the new key with fallback to the legacy. Dual-write is required for rolling-deploy safety -- a controller running pre-rename code observing a pod that carries only skipper/assignment would treat it as unassigned and trigger cleanup. Removal of the legacy write is sequenced for a release boundary after one full deploy plus a deprecation window.

Two typed keys live side by side: LegacyFunctionKey (Name "function", used by every existing emission site this PR touches) and AssignmentKey (Name "assignment", declared but only wired into the K8s annotation write). Subsequent PRs route AssignmentKey through HTTP header dual-parse and telemetry dual-emit, then CLI flags and web URLs.

The proto rename is binary-wire-safe: field tag numbers stay stable, so mixed-version controller / router gRPC interoperates throughout the migration.

Test plan

• dev test -short ./... -- 1216 pass, 34 skipped
• dev lint clean
• dev kube-lint clean
• internal/skipper/testdata/json.golden regenerated for the proto rename; internal/key/testdata/keys.golden byte-identical (legacy emission preserved)
• New table-driven cases in TestAssignmentFromPod cover the four annotation-presence shapes (legacy-only, new-only, both, neither)
• Manual mixed-version verification deferred to staging deploy; static review confirms field-tag-number stability across the rename

[scott-rc]

This change is part of the following stack:

• Rename Function to Assignment and dual-write K8s annotations #138 ◀

_{Change managed by git-spice.}