feat: add copyable no-progress trace

[abhinandanjoshii]

Add an illustrative no-progress trace with a copy control and clear Shadow Mode / Earned Enforcement boundary.

Closes #38

Problem

The Killer Demo makes the allocation decision watchable, but the mechanism behind it is not
quotable. There is no compact form of the no-progress pattern that can be pasted into an issue, a
blog post or a reproduction report. demos/killer-demo/RESULTS.md already tells the reader that
"the no-progress repetition sequence shown in the HTML is an explicitly labeled runtime-pattern
illustration" ,but no such sequence was in the HTML.

Solution

A new #no-progress section on the demo page, plus a nav entry, containing a plain-text trace:

• build_no_progress_trace() replays the shipped NoProgressDetector over a fixed synthetic
  observation sequence. The reason codes are produced by the control, not written by hand, so the
  snippet cannot drift from the mechanism it illustrates;
• each line carries the semantic action, so the sequence reads as the issue describes it: a patch
  produces new evidence, a verification produces new evidence, the same verification repeats
  against unchanged state, and the repetition becomes a no-progress candidate —
  NO_PROGRESS_CLEAR, NO_PROGRESS_CLEAR, NO_PROGRESS_OBSERVED,
  NO_PROGRESS_ENFORCEMENT_ELIGIBLE, with the detector's own reason strings and the
  max_same_evidence_completions threshold printed in the header;
• the snippet header labels it an illustrative deterministic sequence, "not provider telemetry, not
  an enforcement benchmark", consistent with the existing TRUTH BOUNDARY card;
• the closing lines keep the authority boundary explicit: Shadow Mode records the stop
  recommendation and still executes the action; Earned Enforcement is separate and is not granted
  by anything on the page.

Copy affordance and accessibility:

• a COPY TRACE button using navigator.clipboard.writeText with a selection-based fallback and
  no network call (the existing "fetch(" not in js assertion still holds);
• result announced through a role="status" aria-live="polite" line rather than only visually,
  cleared on each attempt so a repeated copy is re-announced;
• the fallback path restores focus to the copy button, so a keyboard user does not lose their
  place;
• the trace is a focusable scroll region (tabindex="0") with a visible :focus-visible ring;
• PRE added to the tag list the global key handler ignores, so Space and → keep driving the
  race and do not fire while the trace region has focus;
• no new animation or transition; the existing prefers-reduced-motion block still covers the
  section.

User impact

The demo page gains one section. No behaviour change to the policy, ledger, adapters or CLI.

Validation

• Tests added or updated
• ruff format --check .
• ruff check .
• mypy src/marginal
• pytest -q
• Documentation updated
• Public API, schemas, examples, roadmap, and changelog are consistent
• Performance claims are reproducible and honestly labeled
• New persisted fields have an explicit privacy classification
• Strict telemetry contains no free text or unreviewed metadata
• Privacy fixtures use synthetic identifiers and no real user data

Security and compatibility

No new public export, no schema change, no serialization change. build_no_progress_trace and
render_no_progress_trace live in marginal.killer_demo alongside the other renderers and are not
re-exported from marginal/__init__.py, matching the existing surface.

Privacy review

• Privacy profile(s) affected: none
• New or changed persisted fields: none
• Quasi-identifiers considered: every value in the snippet is invented — synthetic state and
  evidence digests (w#4f2a, e#2d77), a generic semantic action, no path, repository, model,
  customer or run identifier.
• Key-management or export implications: none

[SignalLayerLabs]

Please fix the CI / Test failed and than we can push it

[SignalLayerLabs]

Please fix the failed CI / tests