feat(scheduler-core): implement REPL execution control (task 12.1)

Adds the interactive simulation REPL's execution-control slice (Req 5
AC 22): a single-threaded input loop (readline-based) supporting
step [N], continue, quit, and help, pausing before the first replay
event and blocking the driver loop while active.

Review round 1 raised two findings, both resolved here:

- GNU readline (GPL) was linked directly into xce_scheduler, a shipped
  production daemon. Switched the whole project to libedit (BSD) for
  line editing -- scheduler and controller (xce_test) both now use
  editline/readline.h, API-compatible (readline()/add_history()).

- Getting a clean regression run after the libedit swap required a
  full clean rebuild, which exposed a genuine pre-existing SIGSEGV
  (bug-1504): SnapshotData.internalState was a capnp Data field, and
  this codebase's capnpc-c backend silently no-ops Data-field
  encoding without zero-initializing the caller's stack struct,
  leaving an uninitialized capn_ptr written onto the wire. Fixed by
  switching internalState to List(UInt8) (with a new internalStateLen
  wire field), matching the working FetchFileReply.content/bytesRead
  precedent -- no hand-written accessor code needed.

A second, unrelated instance of the same bug class was found in the
controller's WAL-catchup path (WalCatchupEntry.payload) and
deliberately left unfixed as bug-1505, out of scheduler-core's
boundary; needs its own project-wide Data-field audit.

Scheduler suite: 34/34 clean. RED-phase reproduction recorded in
tasks.md (repl_should_pause() step-budget check disabled -> 2/4
tests fail as expected -> reverted -> 34/34 green).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JPPXeKSUXUqcqvzNXY1rzJ

src/controller/testsuite/test.c

@@ -10,8 +10,9 @@
 #include <time.h>
 #include <unistd.h>
 
-#include <readline/history.h>
-#include <readline/readline.h>
+#include <editline/readline.h> /* libedit (BSD) -- provides readline()/
+                                 * add_history(), same as GNU readline's
+                                 * split readline.h/history.h pair */
 
 #include <cjson/cJSON.h>
 #include <cjson/s2j.h>