Update periodic_tasks example to ic-cdk-timers v1.0.0 API

Problem

The rust/periodic_tasks/timer example uses ic-cdk-timers = "0.6" and the old closure-based API. The ic-cdk-timers v1.0.0 release (2025-11-13) introduced breaking changes to function signatures.

What needs updating

Dependencies (timer/Cargo.toml)

• ic-cdk 0.12 → 0.19
• ic-cdk-macros 0.8 → current
• ic-cdk-timers 0.6 → 1.0

Code (timer/src/lib.rs)

set_timer_interval signature changed — now takes FnMut() -> impl Future instead of FnOnce():

// Old (line 71):
let timer_id = ic_cdk_timers::set_timer_interval(secs, periodic_task);

// New:
let timer_id = ic_cdk_timers::set_timer_interval(secs, || async { periodic_task() });

ic_cdk::spawn() comment is obsolete (lines 75-77) — set_timer now takes futures directly, no spawn wrapper needed.

dfx references in doc comments (lines 53, 65, 82, 96) — dfx is deprecated, should use icp CLI.

Heartbeat example (heartbeat/Cargo.toml)

• Same outdated ic-cdk/ic-cdk-macros versions

Project-level

• dfx.json at project root — should be migrated to icp.yaml

References

• ic-cdk-timers v1.0.0 changelog
• Found while updating developer-docs PR #9 (timers guide)

Start with rust/periodic_tasks/timer/Cargo.toml and src/lib.rs, then inspect heartbeat/Cargo.toml and the project-root dfx.json. Compare the timer code with the ic-cdk-timers v1.0.0 changelog and verify the documented API and CLI references. Done means both examples use the requested dependency versions and API, doc comments use icp, and the project configuration is migrated to icp.yaml.