Adds chart patterns doc for resource verdicts

[johnnyt]

Why

A chart that reaches an external resource has to route somewhere, explicitly, when the host reports that resource unavailable - a host's external connection or credential may be paused or revoked. The engine's posture (errors are events, no silent defaults) puts that decision in the document, but nothing embedder-facing showed the two canonical shapes. st-3twi asks for a patterns doc plus a runnable example pinning both.

What

• docs/chart-patterns.md: an embedder-facing guide to the two reactions - park/retry for a temporarily unavailable resource (retry loop via <send delay>, with the retry budget and backoff visible as states in the document, not hidden in handler code) and fail-fast for a permanently unavailable one (explicit arrow to a failed final state). The guide is explicit that the temporary/permanent verdict is the HOST's, delivered as events (distinct names, or one event routed on payload with cond); the chart only routes. Cites the existing seams instead of re-arguing them: docs/extending.md/ADR-0051 for error.execution vs error.communication, docs/durable-timers.md (ADR-0054/0059) for park delays that outlive the process, ADR-0004 for the errors-are-events posture.
• test/statifier/chart_patterns_test.exs: the guide's example chart, verbatim, driven through Statifier.Testing.Case in both arms - three pauses exhaust the budget into the failed final state; a revoked-style verdict fails fast from requesting - so the doc cannot drift from engine behavior.
• README gains a pointer to the new guide alongside the extending/durable-timers/testing-charts entries.

Notes

• Deliberately placed as a new doc rather than an docs/extending.md section: Adds Invoke.Handler conformance case #216 (st-718j) adds an extending.md section on the same page region, and this content is chart-authoring guidance, not handler-writing guidance - the two compose with no textual overlap (this branch does not touch docs/extending.md).
• Both new tests carry verified sabotage notes (schedule-instruction drop in Statifier.Session; enabled-set drop in Statifier.Interpreter.Selection.select_transitions/2), each run red and reverted green.
• Gate: full mix quality green (2,479 tests, 96.5% coverage) and mix quality --profile merge green; the ADR judge skipped cleanly (no files in a judged scope). Skips were not-applicable only (gettext).
• No changelog fragment: docs and test harness only, no public-surface change.

Closes: st-3twi