chore(test-seams): move _test*/_force* seams from public to internal

Per audit-2026-05-04 P2-14: VoiceController._forceState,
_testFireSpeechEnd, _testCurrentSttSessionId were declared `public`
solely so XCTest could see them. Same for MCPClient._testHandle and
MCPServerHandle._testProcessIdentifier. These leaked test-only entry
points into each package's public API — anyone importing Voice could
call _forceState(.speaking) from production code.

Voice (4 seams): public → internal. Voice tests already use
@testable import Voice, so no test-side change required.

MCP (2 seams): public → @_spi(Testing) public. MCP tests opt in
via @testable @_spi(Testing) import JarvisMCP. The Harness package
(non-test) consumes _testHandle / _testProcessIdentifier for the
MCPCrashRunner SIGKILL recipe — opt in via @_spi(Testing) import.
@testable alone does NOT expose @_spi symbols — the SPI must be
explicitly imported, which is the desired discipline (each consumer
acknowledges they're reaching past the public ABI).

6 test seams total moved off the public ABI. Voice + MCP tests
pass (87 + 85). Harness builds clean.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

packages/Harness/Sources/Harness/Runners/MCPCrashRunner.swift

@@ -1,6 +1,6 @@
 import Foundation
 import Darwin
-import JarvisMCP
+@_spi(Testing) import JarvisMCP
 import MCP
 
 /// MCPCrashRunner — Plan 08-03 Task 2.

packages/MCP/Sources/MCP/MCPClient.swift

@@ -189,7 +189,10 @@ public actor MCPClient {
     #if DEBUG
     /// Test-only accessor into the registry. Used by restart-mutex tests that
     /// need to SIGKILL the helper PID directly. NOT for production use.
-    public func _testHandle(named name: String) -> MCPServerHandle? {
+    /// Gated by `@_spi(Testing)` so callers must opt in via
+    /// `@_spi(Testing) import JarvisMCP` (used by tests via `@testable` and
+    /// by the eval `Harness` package). Plain `import JarvisMCP` cannot see it.
+    @_spi(Testing) public func _testHandle(named name: String) -> MCPServerHandle? {
         registry[name]
     }
     #endif

packages/MCP/Sources/MCP/MCPServerHandle.swift

@@ -338,7 +338,8 @@ public actor MCPServerHandle {
 
     #if DEBUG
     /// Returns the helper's PID for tests that need to SIGKILL the child directly.
-    public func _testProcessIdentifier() -> Int32 {
+    /// Gated by `@_spi(Testing)` — see `MCPClient._testHandle` for rationale.
+    @_spi(Testing) public func _testProcessIdentifier() -> Int32 {
         process?.processIdentifier ?? -1
     }
     #endif

packages/MCP/Tests/MCPTests/MCPRestartTests.swift

@@ -11,7 +11,7 @@
 import XCTest
 import Darwin
 import MCP
-@testable import JarvisMCP
+@testable @_spi(Testing) import JarvisMCP
 
 final class MCPRestartTests: XCTestCase {
     private var helperBinary: URL!

packages/Voice/Sources/Voice/VoiceController.swift

@@ -229,12 +229,14 @@ public actor VoiceController {
     // MARK: - Test seams
 
     /// Force the controller into a specific state. Tests only.
-    public func _forceState(_ newState: VoiceState) async {
+    /// `internal` + `@testable import Voice` keeps this out of the public
+    /// ABI so production callers (App, Harness) cannot reach in.
+    internal func _forceState(_ newState: VoiceState) async {
         state = newState
     }
 
     /// Simulate a VAD speech-end + STT finalization with given text. Tests only.
-    public func _testFireSpeechEnd(text: String = "") async {
+    internal func _testFireSpeechEnd(text: String = "") async {
         await handleSTTFinalized(text: text)
     }
 
@@ -242,13 +244,13 @@ public actor VoiceController {
     /// finalize path with an explicit `sessionId` so tests can simulate a
     /// finalize callback from a prior session arriving after the next
     /// session has started.
-    public func _testFireSpeechEnd(text: String, sessionId: UInt64) async {
+    internal func _testFireSpeechEnd(text: String, sessionId: UInt64) async {
         await handleSTTFinalized(text: text, sessionId: sessionId)
     }
 
     /// P1-3 stale-text seam (tests only). Returns the current session id so
     /// tests can capture it before triggering a session boundary.
-    public func _testCurrentSttSessionId() -> UInt64 {
+    internal func _testCurrentSttSessionId() -> UInt64 {
         sttSessionId
     }