Add reliable programmatic model load and save behavior

[burhop]

Summary

This change makes the existing embedded CAD API reliable for programmatic model loading and saving:

• missing or invalid model loads reject with useful errors instead of opening a blocking alert() and returning;
• CadEmbed.saveCurrentTo() saves to an explicit destination without dialogs or window.confirm();
• save conflicts, explicit overwrite, cancellation, persistence verification, and rollback are handled deterministically;
• timed-out or aborted requests cancel the frame-side operation so they cannot write later;
• the existing interactive toolbar save behavior remains unchanged.

Why each file changed

• src/UI/cad/CadEmbed.ts — exposes saveCurrentTo(), accepts cancellation, sends cancel messages on abort/timeout, and cleans up pending requests.
• src/UI/cad/CadFrameApp.ts — routes explicit save requests, owns a per-request AbortController, and cancels frame-side work when requested or disposed.
• src/UI/fileManagerWidget.ts — adds the non-interactive save path with explicit conflict/overwrite behavior, verification, and rollback; programmatic load failures now reject instead of displaying alerts.
• src/UI/cad/nonInteractiveSave.ts — isolates destination validation, abort handling, persistence verification, and rollback logic.
• src/services/modelLoadErrors.ts — provides consistent errors for missing and invalid model records.
• apiExamples/Embeded_CAD_Integration_Test.ts — covers explicit save, conflicts, overwrite, cancellation, ghost-write prevention, and pending-request cleanup in the browser.
• scripts/nonInteractiveSave.test.ts — covers save validation, destination planning, cancellation, verification, and rollback without requiring the full browser toolchain.
• src/tests/test_file_manager_programmatic_load_errors.ts — verifies a missing programmatic model load rejects without calling alert().
• src/tests/tests.ts — registers the load-error regression test in the existing Node/browser harness.
• docs/developer/embedding/cad-embed.md — documents the interactive and non-interactive save APIs, options, results, cancellation, and rollback behavior.

Compatibility and scope

• The existing interactive saveCurrent() toolbar behavior is preserved.
• No geometry, feature, topology, serialization, or rendering behavior changes.
• No dependency or lockfile changes.
• No server, protocol, headless packaging, or platform bootstrap changes.
• The new embedding save method is additive.

Validation

• pnpm install --frozen-lockfile — passed
• pnpm typecheck — passed
• changed-file ESLint — passed
• node --import tsx --test ./scripts/nonInteractiveSave.test.ts — 6/6 passed
• pnpm exec tsx ./src/tests/tests.ts test_file_manager_missing_model_rejects_without_alert — passed
• pnpm exec vite build --config vite.config.kernel.ts — passed
• CadEmbed browser integration — 25 passed, 1 optional load test skipped because no model path was supplied
• http://127.0.0.1:5196/ — HTTP 200 during validation
• http://127.0.0.1:5196/src/CAD.ts — HTTP 200 during validation

The repository-wide pnpm lint command still reports the pre-existing no-unreachable error in unchanged scripts/capture.ts:1149; this PR does not modify that file. The contributor has reviewed and accepted the repository license's pull-request copyright-assignment condition.