docs(runtime): document host mode-info contract for registry convergence

Summary:
Update runtime module documentation to match the new host mode-info contract used for registry synchronization and additive runtime capability derivation.

Why:
The implementation now relies on the richest available host ServerModeInfo snapshot so backend Rust can preserve dedicated embedding-sidecar facts during runtime-registry convergence. The README contracts needed to reflect that supported path and warn against narrower core-gateway-only sync flows.

What changed:
- document in crates/pantograph-embedded-runtime/src/README.md that hosts with extra producer facts should pass a richer ServerModeInfo snapshot into the hosted runtime constructor
- document that registry reconciliation should consume the richest available producer snapshot contract instead of bypassing it with a narrower core-gateway-only helper
- document in src-tauri/src/llm/README.md that Tauri registry synchronization must use the richer wrapper mode_info() payload so embedding-sidecar facts are preserved

Verification:
- documentation-only change reviewed against the current runtime and registry implementation

Author: MrScripty

crates/pantograph-embedded-runtime/src/README.md

@@ -156,6 +156,10 @@ let runtime = EmbeddedRuntime::with_default_python_runtime(
 - Hosts may optionally inject a shared runtime registry; when present, session
   runtime load/unload lifecycle is translated into registry reservation
   acquire/release operations.
+- Hosts that own additional producer snapshots beyond the core
+  `inference::InferenceGateway` may pass a richer `ServerModeInfo` snapshot
+  into the hosted runtime constructor so backend Rust can derive registry
+  observations and additive runtime capabilities from one contract.
 - Direct embedded workflow runs may also reconcile Python-sidecar execution
   snapshots into that shared registry so producer-specific runtime facts do not
   depend on Tauri-only diagnostics paths.
@@ -184,5 +188,10 @@ let runtime = EmbeddedRuntime::with_default_python_runtime(
   consumers; a future `RuntimeRegistry` may compose them with admission or
   residency state, but this crate must not silently fold policy-level decisions
   into those producer contracts.
+- Registry reconciliation should consume the richest available producer
+  snapshot contract, typically a host-owned `ServerModeInfo`; callers should
+  not bypass that contract with a narrower core-gateway-only helper when
+  additional producer facts such as the dedicated embedding sidecar are
+  available.
 - If the descriptor contract changes, this directory must regenerate its README
   contract text and add ADR coverage if the compatibility boundary expands.

src-tauri/src/llm/README.md

@@ -117,6 +117,9 @@ app.manage(gateway);
   single producer-specific runtime snapshots into backend-owned registry
   observations, but it must not become the owner of lifecycle or retention
   policy.
+- When this directory synchronizes registry state from the shared gateway, it
+  must use the richer Tauri `mode_info()` snapshot rather than the narrower
+  core-gateway view so dedicated embedding-sidecar facts are not dropped.
 - Command payloads emitted from this directory are transport wrappers around
   backend/runtime contracts, not a separate policy schema.
 - Health/recovery overlays may add host-only fields, but they must not mutate