Align AXIS control flow and fix pause and program rendering

Document source UI gates, explicit native live-plot clearing, and remaining parity boundaries. Fix pause/resume transport continuity, repeated stepping, coordinate previews, and spindle override state. Optimize large program snapshots and rendering, and cover the control flows with native Tcl, WASM, and browser checks.

Validation: 22 native tests and source/browser checks pass. Worker/App regression: 113 of 123 pass; 10 pre-existing failures remain documented with no new failures.

web/worker/test_xyzac_program_replace.mjs

@@ -3,6 +3,7 @@ import { readFile } from "node:fs/promises";
 import createModule from "../../build-wasm/linuxcnc-sim-core.js";
 import { loadSimulationCase, toWorkerConfiguration } from "../workspace/sim-case-loader.mjs";
 import { SimSession, CONTEXT_ID } from "./sim-session.mjs";
+import { fnv1a64 } from "./replay.mjs";
 
 const CASE = "linuxcnc/configs/sim/axis/vismach/5axis/table-rotary-tilting/xyzac-trt.ini";
 const fetchImpl = async (url) => {
@@ -29,6 +30,38 @@ assert.equal(second.payload.program.path, "second.ngc");
 assert.notEqual(second.payload.program.source_hash, first.payload.program.source_hash);
 assert.equal(second.payload.panel_snapshot.visual.toolpath.length, 1);
 
+const epoch = () => session.livePathEpoch;
+const before = epoch();
+const source = "G21 G90\nG1 X2 Y2 Z3 F100\nM2\n";
+const replaced = send("PROGRAM_LOAD", { source, path: "web-replacement.ngc", clear_live_plot_on_replace: true });
+assert.equal(replaced.kind, "COMMAND_RESULT");
+assert.equal(replaced.payload.visual_clear, undefined);
+assert.equal(epoch(), before, "changed file retains the native trace, even from an old caller");
+assert.equal(replaced.payload.program.source_hash, fnv1a64(source));
+assert.equal(session.xyzacProgram.snapshot().source_hash, fnv1a64(source));
+assert.equal(send("PROGRAM_LOAD", { source, path: "web-replacement.ngc", clear_live_plot_on_replace: true }).kind, "COMMAND_RESULT");
+assert.equal(epoch(), before, "unchanged reload preserves live history");
+const failedReplacement = send("PROGRAM_LOAD", { source: "G999\n", path: "bad-web.ngc", clear_live_plot_on_replace: true });
+assert.equal(failedReplacement.kind, "ERROR");
+assert.equal(epoch(), before, "failed parse does not clear history");
+const native = send("PROGRAM_LOAD", { source, path: "native.ngc" });
+assert.equal(native.kind, "COMMAND_RESULT");
+assert.equal(epoch(), before, "default source PROGRAM_LOAD preserves history");
+send("POWER", { active: true }); send("HOME", { all: true }); send("PROGRAM_RUN");
+const blocked = send("PROGRAM_LOAD", { source, path: "blocked.ngc", clear_live_plot_on_replace: true });
+assert.equal(blocked.kind, "ERROR");
+assert.equal(epoch(), before, "gate rejection does not clear history");
+send("PROGRAM_ABORT");
+const edited = send("PROGRAM_LOAD", { source: source.replace("X2", "X3"), path: "native.ngc", clear_live_plot_on_replace: true });
+assert.equal(edited.kind, "COMMAND_RESULT");
+assert.equal(epoch(), before, "edited text retains the native live trace");
+assert.equal(edited.payload.program.source_hash, fnv1a64(source.replace("X2", "X3")));
+const cleared = send("VISUAL_CLEAR");
+assert.equal(cleared.kind, "COMMAND_RESULT");
+assert.equal(epoch(), before + 1);
+assert.deepEqual(cleared.payload.before, cleared.payload.after, "clear must not change CNC state");
+assert.deepEqual(cleared.payload.panel_snapshot.visual.toolpath, edited.payload.panel_snapshot.visual.toolpath);
+
 console.log(JSON.stringify({
   schema: "xyzac-program-replacement.v1",
   status: "verified",