feat(runtime): add SGLang runtime backend (#974)

[joryirving]

What

Adds sglang as a first-class InferenceService.Spec.Runtime value,
alongside llamacpp / vllm / tgi / personaplex / generic.

A new typed SGLangConfig (with nested SGLangSpeculativeConfig) covers
sharding, memory, batching, quantization/KV-cache, attention, agentic glue
(tool/reasoning parsers, chat template), speculative decoding (EAGLE /
EAGLE3), and LoRA basics — mirroring the vLLM pattern. GPU vendor (NVIDIA
vs AMD) drives image selection, so the AMD ROCm image is picked automatically
when model.spec.hardware.gpu.vendor is amd.

SGLang launches via python3 -m sglang.launch_server (mirroring how
personaplex wraps a Python module). Probes target /health_generate for
startup + readiness (runs a token, accurate) and /health for liveness
(cheap). Startup tolerates 180 failures (~30 min) for cold-start model load.

Why

Fixes #974

Headline motivation is SGLang's RadixAttention automatic prefix caching.
A foreman agent's tool loop re-sends the same large shared prefix every turn
(system prompt + tool definitions + issue/repo context); SGLang serves that
shared prefix from cache instead of reprocessing it, which materially cuts
latency and raises throughput for the multi-turn, high-concurrency pattern
the foreman-agent generates. Today an operator who wants SGLang has to fall
back to the generic runtime and hand-write container config — losing the
auto-generated args, mode handling, and health wiring the first-class
backends get.

How

• api/v1alpha1/inferenceservice_types.go: SGLangConfig +
  SGLangSpeculativeConfig structs, SGLangConfig field on
  InferenceServiceSpec, sglang added to the runtime enum.
• internal/controller/runtime_sglang.go: SGLangBackend implementing
  RuntimeBackend + CommandBuilder + HPAMetricProvider +
  EnvBuilder. BuildArgs emits base flags + --served-model-name +
  typed fields in declaration order + auto-derived --tp + --is-embedding
  for mode + extraArgs last.
• internal/controller/runtime_sglang_args.go: free-function helpers,
  one per flag. Mirrors runtime_vllm_args.go structure.
• internal/controller/runtime.go: case RuntimeSGLANG in
  resolveBackend.
• internal/controller/deployment_builder.go: resolveRuntimeImage
  picks ROCm image for AMD GPUs; otherwise CUDA.
• make manifests + make chart-crds regenerated CRDs.
• docs/contributors/adding-a-runtime.md, docs/site/concepts/comparison.md,
  docs/site/guides/model-matrix.md updated.

Out of scope (B3 followup): LoRA hot-loading, speculative perf tuning
(--speculative-accept-threshold-*), data-parallel rendezvous,
PodMonitor for /metrics, and remaining minor SGLang flags. Tracked as a
separate issue.

Checklist

• Tests added/updated (TestSGLangBuildArgs table-driven +
  TestSGLangBuildCommand + TestSGLangProbes + TestSGLangBuildEnv +
  TestValidateSGLangConfig + dispatch + image-resolution tests)
• make test passes locally
• make lint passes locally
• make lint-all passes locally (cross-arch)
• Commit messages follow conventional commits
• All commits are signed off (git commit -s)
• AI assistance disclosed: this PR was produced by an AI coding agent
  working with a repo collaborator. The agent followed the
  docs/contributors/adding-a-runtime.md pattern, the design spec at
  docs/superpowers/specs/2026-07-10-sglang-runtime-backend-design.md
  (local agent scratch, not committed), and AGENTS.md conventions.
• Documentation updated

[codecov]

Codecov Report

❌ Patch coverage is 70.71651% with 94 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
api/v1alpha1/zz_generated.deepcopy.go	0.00%	91 Missing and 1 partial ⚠️
internal/controller/runtime_sglang.go	98.07%	1 Missing and 1 partial ⚠️

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