feat(scheduler-core): implement REPL snapshot capture and simulation parameter commands (task 12.3)

Adds `snapshot [FILE]` and `set speed|output|verify ...`/`info config`
(Req 5 AC 22 "Snapshots"/"Simulation parameters") to the interactive
simulation REPL.

`snapshot` reuses sim_record.c's existing sim_record_write_header_and_
snapshot() (task 11.5) rather than reimplementing capnp encoding, so
the produced file is genuinely loadable via a fresh --sim invocation
-- the task's own Observable. `set speed`/`set output`/`set verify`
mutate fields sim_loop.c and core_clock.c already read fresh on every
event/session, so no new hook points were needed and sim_loop.c stays
untouched at exactly 800/800 lines (this project's hard per-file cap).

New repl_params.c is a fourth sibling in the repl.c/repl_break.c/
repl_info.c split (task 12.2's precedent) -- neither existing file had
headroom left under the 800-line cap for this task's own logic.

Scheduler suite: 36/36 clean (was 35/35). Review independently
reproduced the RED-phase probe and additionally verified the
snapshot round-trip Observable is non-vacuous.

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

src/scheduler/testsuite/meson.build

@@ -560,6 +560,9 @@ sched_sim_isolation_test = executable(
              '-DXCE_SIM_REPL_INTERNAL_H_PATH="'
              + (meson.current_source_dir() / '..' / 'src' / 'repl_internal.h')
              + '"',
+             '-DXCE_SIM_REPL_PARAMS_C_PATH="'
+             + (meson.current_source_dir() / '..' / 'src' / 'repl_params.c')
+             + '"',
              '-DXCE_SIM_LIB_PATH="'
              + sched_sim_isolation_lib.full_path()
              + '"'],
@@ -708,6 +711,31 @@ sched_repl_breakpoints_inspection_test = executable(
     dependencies: [scheduler_test_deps, libedit_dep],
     install: false)
 
+# Contract test for the interactive simulation REPL's snapshot-capture and
+# simulation-parameter command slice (scheduler-core spec, task 12.3, Req 5
+# AC 22 "Snapshots"/"Simulation parameters"): `snapshot [FILE]` writes a
+# standalone .replay file a FRESH sched_sim_driver_start() can load, landing
+# on the exact position the snapshot was taken at (the task's own central
+# Observable, proved end to end via a forked child + a second, real
+# sched_sim_driver_start() call in the parent); `set speed`/`set output`/
+# `set verify` mutate the parameters `info config` reports. Same white-box
+# #include pattern as sched_repl_breakpoints_inspection_test above (core.c,
+# core_clock.c, core_events.c, core_session.c, decision.c, plugin.c,
+# plugins/builtin_fcfs.c, recording.c, recording_session.c, sim.c,
+# sim_loop.c, sim_record.c, sim_verify.c,
+# snapshot.c/snapshot_wire.c/snapshot_load.c, repl.c, repl_break.c,
+# repl_info.c) PLUS the new ../src/repl_params.c under test. Split into its
+# own file rather than appended to test_repl_breakpoints_inspection.c --
+# that file was already over 1000 lines after task 12.2, and this task's
+# own fixture/assertion surface (a fork+reload round-trip plus five
+# further unit-level command tests) does not belong crammed into it.
+sched_repl_snapshot_params_test = executable(
+    'sched_repl_snapshot_params_test',
+    ['test_repl_snapshot_params.c'],
+    include_directories:[incdir],
+    dependencies: [scheduler_test_deps, libedit_dep],
+    install: false)
+
 # Register as meson tests so `meson test --suite scheduler` runs them all.
 # detect_leaks=0 matches the suite convention (fixtures are not torn down).
 test('scheduler test', scheduler_test, suite: 'scheduler',
@@ -804,3 +832,6 @@ test('scheduler repl execution control test',
 test('scheduler repl breakpoints inspection test',
      sched_repl_breakpoints_inspection_test, suite: 'scheduler',
      env: ['ASAN_OPTIONS=detect_leaks=0'])
+test('scheduler repl snapshot params test',
+     sched_repl_snapshot_params_test, suite: 'scheduler',
+     env: ['ASAN_OPTIONS=detect_leaks=0'])