refactor: split format/buffers.go model from render (#4661)

[psaab]

Summary

Closes #4661.

Extracts the show system buffers row/taxonomy model out of
format/buffers.go into a new buffers_model.go, mirroring the #4656
status_sections.go and #4657 cos_sections.go model-then-render split.
Pure code motion — the rendered output is byte-identical.

Why

Prior CLI/gRPC/REST buffer-status parity bugs came from the two renderers
building the row set independently:

• FormatSystemBuffers (text) — used by CLI (cli_show_system.go) and gRPC (server_show_system.go)
• StructuredSystemBufferRows (structured) — used by the REST endpoint (api/system.go)

Both ran the same three-call sample/row/counter build. This factors that
duplicated build into one shared buildSystemBufferModel returning a
systemBufferModel (rows + counterRows + knownUMEM/knownTX), so a row or
counter added to the taxonomy surfaces in every renderer at once and cannot
drift.

Layout after the split

• buffers_model.go (new): taxonomy label constants, internal
  sample/row/counter/binding-key types, buildSystemBufferModel, the row
  builders, the WARNING/CRITICAL usage classifier, helper-status sampling +
  per-binding fallback, and scope helpers.
• buffers.go (773 → 160 LOC): section-heading constants, exported REST
  row types, exported API, and FormatSystemBuffers reduced to a
  build-then-render orchestrator over two new per-section render helpers
  (writeSystemBufferUtilizationSection / writeSystemBufferCountersSection).

The counter-section leading-blank-line spacing (HasSuffix "\n\n") is
preserved. No symbol collision with status_sections.go / cos_sections.go.

Byte-identity gate

TestFormatSystemBuffersGolden pins the full output of both render modes
(detail=false/true) plus the "unavailable" branch for a fixture exercising
UMEM/TX/CoS aggregate+detail rows, bounded session/flow-cache/neighbor rows,
the WARNING/CRITICAL classes, the high-util warning count, and every status
counter with per-binding detail. The golden fixture was generated by running
the same test on a pristine pre-refactor origin/master worktree; the
refactored code then reproduced it byte-for-byte (passes without -update).
Reuses the package-level updateGolden flag from status_golden_test.go.

Validation

• go build ./... clean
• go vet ./pkg/dataplane/userspace/format/ clean
• gofmt clean
• go test ./pkg/dataplane/userspace/format/ green (incl. the new golden + all existing substring tests)

No module-doc change needed: the format package has no README (matches
#4656/#4657, which touched only _Log.md + the golden test/fixture), and the
split is behavior-identical.

🤖 Generated with Claude Code

https://claude.ai/code/session_015oARShYtiJJ2H4UB4nXGqi

[Copilot]

Pull request overview

Refactors show system buffers formatting to share a single, renderer-agnostic row/counter model between the text (CLI/gRPC) and structured (REST) outputs, eliminating prior drift risk while keeping output byte-identical.

Changes:

• Extracts the buffer row/counter taxonomy + sampling/aggregation logic into a new buffers_model.go model builder (buildSystemBufferModel).
• Reduces buffers.go to exported structured types/APIs plus a build-then-render FormatSystemBuffers with per-section render helpers.
• Adds a golden test + fixture to gate byte-for-byte output stability across detail modes and the “unavailable” branch.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
pkg/dataplane/userspace/format/buffers.go	Converts text/structured renderers to consume a shared model and splits rendering into utilization/counters helpers.
pkg/dataplane/userspace/format/buffers_model.go	New SSOT model: taxonomy labels, sampling, aggregation, row/counter construction, and usage classification.
pkg/dataplane/userspace/format/buffers_golden_test.go	Adds a golden test to enforce byte-identical output across render modes/branches.
pkg/dataplane/userspace/format/testdata/system_buffers.golden	New golden fixture capturing expected output for the new test gate.
_Log.md	Logs the refactor per repository logging rules.