Audit assignment vocabulary in prose and code symbols

[scott-rc]

Phase 5 of the assignment-redesign plan. Phases 1-4 changed the canonical surfaces (Go types, gRPC fields, HTTP headers, K8s annotations, Prometheus labels, OTLP attributes, slog keys, CLI flags, Web UI routes) but the long-tail surfaces lagged behind. This sweep brings them in line: doc-comments across internal/, README.md, the architecture and guide pages, help-text description: tags, one Prometheus Help: string, the assignmentHashIndex const value ("functionHash" -> "assignmentHash"), a handful of stale Test*/Benchmark* identifiers, and the previously-untracked UBIQUITOUS_LANGUAGE.md at the repo root.

A new forbidden-tokens doclint rule (with a TDD-style test in internal/dev/doclint/forbidden_tokens_test.go) catches regressions for FunctionHash, FunctionFromHeader, skipper.Function, skipper.FunctionKey, and functionHash -- five symbol-shaped tokens that have no legitimate post-rename usage. The test also pins the word-boundary edge case so skipper.FunctionKey does not double-match the bare skipper.Function pattern.

fn -- the short form of "function" from before this domain became "assignment" -- is renamed across the Go codebase to Assignment-based names. Parameters and locals use short a to match existing func (a *Assignment) ... receivers; struct fields and compound identifiers use the full word (assignment, assignmentHeader, assignmentFilter, defaultAssignment, etc.). Genuine func-value callbacks in timer.Poll/Loop and the doclint forEach* helpers keep fn, since there it names a function value rather than an Assignment.

docs/content/guides/deploying-functions.md is renamed to deployments.md (and the docs sidebar in cmd/dev/docs.go updated to match), since the page is really about labelling K8s deployments for Skipper to pool -- a more accurate framing than either "Functions" or "Assignments".

Stacked on top of #141 (Phase 4: flat policy surface + 9 wired knobs). Plan: tmp/assignment-redesign/plan.md.

[scott-rc]

This change is part of the following stack:

• Rename Function to Assignment and dual-write K8s annotations #138
  • Dual-emit assignment vocabulary on header, metrics, and telemetry #139
    • Surface assignment vocabulary on CLI flags and Web UI routes #140
      • Add flat policy surface and wire 9 per-tenant knobs #141
        • Audit assignment vocabulary in prose and code symbols #142 ◀

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