feat: grpc 기능 deep 구현

docs/compatibility/grpc-advanced-support-matrix.md

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+# gRPC advanced capability support matrix
+
+Every capability in `:grpc-advanced:*`, its grade, and what it would take to raise it.
+`GrpcAdvancedSupportMatrix` is the machine-readable form; `GrpcAdvancedCapability.defaultGrade`
+carries the same values.
+
+All capabilities are off by default. Flags are `ca-skeleton.grpc.advanced.<capability>.enabled`.
+
+## Grades
+
+| Grade | May start | Production needs a separate approval |
+| --- | --- | --- |
+| `ADVANCED_STABLE` | Yes | No |
+| `EXPERIMENTAL` | Yes | Yes |
+| `WATCH` | No | — |
+| `DISABLED` | No | — |
+
+## Capabilities
+
+| Capability | Flag | Grade | Real infrastructure its evidence needs |
+| --- | --- | --- | --- |
+| Protobuf Edition 2024 | `edition-2024` | `ADVANCED_STABLE` | — |
+| Protobuf Edition 2026 | `edition-2026` | `WATCH` | — |
+| Client streaming | `client-streaming` | `ADVANCED_STABLE` | — |
+| Bidirectional streaming | `bidi-streaming` | `ADVANCED_STABLE` | — |
+| Manual flow control | `manual-flow-control` | `ADVANCED_STABLE` | — |
+| Read-only unary hedging | `hedging` | `EXPERIMENTAL` | — |
+| Custom name resolver | `custom-resolver` | `ADVANCED_STABLE` | — |
+| Custom load balancer | `custom-load-balancer` | `EXPERIMENTAL` | — |
+| Proxyless xDS | `xds` | `EXPERIMENTAL` | xDS control plane |
+| gRPC-Web | `grpc-web` | `ADVANCED_STABLE` | gRPC-Web proxy |
+| Servlet HTTP/2 | `servlet-compat` | `ADVANCED_STABLE` | Servlet container |
+| Spring Integration bridge | `integration-bridge` | `ADVANCED_STABLE` | — |
+| Reactor adapter | `reactor` | `ADVANCED_STABLE` | — |
+| Kotlin coroutine / Flow | `kotlin` | `ADVANCED_STABLE` | Kotlin toolchain |
+| Channelz / CSDS diagnostics | `channel-diagnostics` | `ADVANCED_STABLE` | — |
+
+## What the grades mean here, concretely
+
+**Grade is a statement about the contract, not about a deployment.** Every capability's contract is
+implemented and tested in this repository. What no capability has is evidence from a real deployment:
+`GrpcAdvancedPromotionEvidence` for each one is empty, and no promotion has been granted.
+
+**Four capabilities cannot produce meaningful evidence here at all**, because the infrastructure they
+need is absent. `GrpcAdvancedInfrastructureTestkit.missingInfrastructure` names them, and a suite that
+runs without its infrastructure passes and establishes nothing.
+
+**Kotlin is the sharpest case.** This repository has no Kotlin toolchain, so
+`GrpcKotlinCompatibilityGate.supportableHere()` returns false and always will until one exists. The
+four contract requirements — one schema source shared with Java, coroutine cancellation propagated,
+Flow backpressure inside the Stable buffer bounds, platform evidence types preserved — are checkable
+without a toolchain and are checked. The compile lane is not.
+
+## Promotion thresholds
+
+| To | Soak | Also required |
+| --- | --- | --- |
+| `ADVANCED_STABLE` | 7 days | compatibility evidence, security review, fault evidence, performance evidence, ADR, runbook, real-environment test |
+| Stable default | 30 days | all of the above, plus a dependency, security and operational-cost review |
+
+`WATCH` becomes `EXPERIMENTAL` before anything else. Promotions are independent: promoting one
+capability changes no other's grade.