[Compute Separation][HT9] Expose the one-worker link and acceptance endpoint

## Roadmap mapping

| Field | Value |
|---|---|
| Roadmap ID | `HT9` |
| Lane | Host |
| BYOC milestone | M1 |
| Parent | #11965 |
| Depends on | `HT4`, `HT8b`, `HT13` / #11971 |
| Blocks | `HT10-M1+HT13B-M1` |
| Pull request | One PR |

## Why this PR exists

Everything before HT9 can only be driven inside the process. The platform must link a worker through an HTTP route. This PR adds one compute-only thin adapter translating the frozen HT13 request into one HT4 registry link and returning definitive acceptance after handshake/registration.

## Source material

- `platform-control-apis.md`, worker linking.
- `ga-plan.md`, link contract and compute-only DTO boundary.
- Roadmap HT9 and incremental link/control milestone.
- HT13 / #11971.
- [internal BYOC #117](https://github.com/serverless-paas-balam/FlexConsumption-BYOC/issues/117) operation/epoch replay and [internal BYOC #219](https://github.com/serverless-paas-balam/FlexConsumption-BYOC/issues/219).
- Prototype `WorkerController`/`ExternalWorkerInfo` are validation/status references only; do not port placement, standby branch, or old connection manager.

## Current state

HT4 links internally; HT8b starts ScriptHost on first completed link; compute executable exists. No external route, DTO, validation, status mapping, replay handling, or acceptance event exists. Standard Host must remain route-free.

## Proposed implementation

1. Confirm method/route/payload/status/replay/acceptance marker in HT13 before merge.
2. Add compute-owned `WorkerLinkRequest`/response DTOs only in compute product; never shared Grpc/Server/Script/standard WebHost.
3. Map the route from compute composition using minimal APIs or compute-only MVC application part. Record choice and prove standard route absence.
4. Validate missing/blank identity, missing body, route/body mismatch, and invalid gRPC/HTTP endpoints with deterministic `400`. Use ordinal identifier comparison.
5. Apply [internal BYOC #117](https://github.com/serverless-paas-balam/FlexConsumption-BYOC/issues/117) admission/replay rules: exact replay idempotent; stale/conflicting/terminal admission rejected without mutation; failed removed entry retryable as frozen by HT13.
6. Translate to HT4 `WorkerLinkContext`, call `LinkAsync`, honor request cancellation, and return success only after connection, `StartStream`, init handshake, and channel registration—not ScriptHost/capacity completion.
7. Map connection/unavailable versus admission/state failures to HT13 status codes and leave no orphaned entry.
8. Emit exact structured acceptance/rejection event with worker ID, operation/epoch, reason, and elapsed time for [internal BYOC #219](https://github.com/serverless-paas-balam/FlexConsumption-BYOC/issues/219) evidence.

### Ownership and lifecycle

Endpoint owns translation/validation/status/replay adapter only. Registry owns channels; HT8b owns ScriptHost startup. Root compute services expose route across ScriptHost restarts. Registry provides same-worker serialization; endpoint adds no competing lock.

## Tests and evidence

- happy link via test server -> success, ready registry entry, HT8b observes first link;
- complete validation ladder;
- exact/stale/conflicting/terminal replay matrix;
- unreachable endpoint -> unavailable, no leaked entry, later retry succeeds;
- cancellation mid-link cleans resources;
- exact acceptance/rejection event fields;
- standard route table has no worker-link route and shared assemblies contain no DTO;
- HT14 service graph/publish/deps/startup evidence.

## Explicit exclusions

No list/status/query, unlink/drain/stop/eviction/reconnect, capacity/slots/runtime-state, auth/key enforcement, specialization/Standby, host.json/link config, standard controller changes, multi-worker admission, billing, extension gRPC, or assembly streaming.

## Acceptance criteria

- Frozen compute route links one worker and succeeds only after handshake/registration.
- Validation and replay semantics are deterministic and non-mutating on failure.
- Unavailable maps correctly and leaves retryable state.
- Acceptance event matches contract.
- Route/DTO are absent from standard/shared products.

## PR stop condition

Stop when the compute-only link endpoint fully implements HT13/[internal BYOC #117](https://github.com/serverless-paas-balam/FlexConsumption-BYOC/issues/117) acceptance/replay semantics and is provably absent from standard Host. Do not add unlink/drain/status, auth, or capacity.