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/schema/controller.capnp

@@ -1109,10 +1109,23 @@ struct SchedProject $C.mapname("sched_snapshot_project_t") {
 # session/iterator state) with no capnp-visible structure -- the core owns
 # its byte layout; internalVersion is a format tag so a future incompatible
 # internal layout is rejected rather than misinterpreted (this task's
-# Observable). Like every other Data field in this schema (see
-# ReplayHeader.configHash above), internalState's generated encode_/decode_
-# codec is a documented no-op -- real extraction is hand-written
-# low-level-accessor code, task 9+'s job, same pattern.
+# Observable).
+#
+# internalState is List(UInt8), NOT Data (task 12.1 review finding, fixed):
+# every OTHER Data field in this schema (e.g. ReplayHeader.configHash) has
+# its generated encode_/decode_ codec silently no-op'd by this codebase's
+# capnpc-c setup -- the generated encode_SnapshotData_ptr()'s local `struct
+# SnapshotData d;` is never zero-initialized, so a no-op'd Data field left
+# capn_setp() writing an UNINITIALIZED stack capn_ptr onto the wire (real,
+# reproducible SIGSEGV in write_SnapshotData, deterministic per call site's
+# stack layout -- configHash happened to get lucky garbage, internalState
+# did not). List(UInt8) sidesteps the whole no-op class: this same capnpc-c
+# backend DOES correctly auto-generate real encode/decode for List(UInt8)
+# (proven precedent: FetchFileReply.content/bytesRead above -- a genuine
+# capn_new_list8()/capn_set8() loop, not a comment placeholder), so no
+# hand-written low-level accessor code is needed here at all. Same
+# maplistcount convention as content/bytesRead: internalStateLen (@11,
+# below) carries the byte count as its own real wire scalar.
 struct SnapshotData $C.mapname("sched_snapshot_data_t") {
   config          @0: Conf4Sched;
   jobs            @1: SchedJobData;
@@ -1123,8 +1136,9 @@ struct SnapshotData $C.mapname("sched_snapshot_data_t") {
   users           @6: List(SchedUser) $C.maplistcount("n_users");
   projects        @7: List(SchedProject) $C.maplistcount("n_projects");
   prngSeed        @8: UInt64 $C.mapname("prng_seed");
-  internalState   @9: Data $C.mapname("internal_state");
+  internalState   @9: List(UInt8) $C.mapname("internal_state") $C.maplistcount("internal_state_len");
   internalVersion @10: UInt32 $C.mapname("internal_version");
+  internalStateLen @11: UInt32 $C.mapname("internal_state_len");
 }
 
 struct SnapshotEntry $C.mapname("replay_snapshot_entry_t") {