Harden engine error handling; 4x tests (24->136) and demos (5->20)

[cognis-digital]

Summary

Hardens the engine's error handling and roughly 4×s the tests and demos, with no change to the public API.

Engine hardening (cyclework/engine.py)

Three real fixes, all surfaced as the existing Status.ERROR / TypeError / ValueError contract:

1. seed() failures now become Status.ERROR. Previously a raising seed escaped the engine as a raw exception — inconsistent with check/revise, both of which were already trapped. The seed is a stage too; it now fails the same honest way (with an empty trace, since nothing ran).
2. check() is guarded to return a Verdict. Returning a bare bool/None/tuple — the single most common misuse — used to surface deep in the loop as a cryptic AttributeError: 'bool' object has no attribute 'ok'. It now produces a clear, stage- and iteration-named TypeError (caught as ERROR in run(), raised directly in cycle()).
3. Constructor validation up front. Non-callable check/revise/seed/on_iteration raise TypeError; min_delta < 0 raises ValueError (joining the existing max_iterations/patience checks).

Tests: 24 → 136

New files test_engine_edge.py, test_verdict_trace.py, test_examples_edge.py cover budget boundaries, plateau/min_delta corners (incl. the strict-> boundary), per-stage error surfacing, the verdict-type guard, config validation, best-tracking with unscored candidates and ties, the cycle() streaming form, the full Verdict/Iteration/Result data model, and example convergence/divergence/error paths.

Demos: 5 → 20

Added scenarios 6–20 (error recovery, gradient descent, multi-rule normalizer, misuse guards, Collatz+budget, seed preprocessing, plateau early-stop, streaming audit log, any-state, retry-with-backoff, constraint repair, best-of non-convergent search, iterative summarizer, run() vs cycle(), Newton vs bisection). run_all.py, tests/test_demos.py, docs/DEMOS.md, docs/ARCHITECTURE.md, and README.md updated. Every demo drives the real API, touches no network, and exits 0.

Verification

$ pytest -q
136 passed
$ PYTHONUTF8=1 python demos/run_all.py   # exit 0

The public surface (Engine, Verdict, Result, Iteration, Status, cyclework.examples) is unchanged — existing code keeps working; the changes only make previously-undefined behavior explicit.