Add Pico2 W educational lessons and timestamp MQTT diagnostics

[mot-yelraf]

Summary

Add a 13-lesson educational series under docs/syllabus-lessons, linked before Quick start in the root README. The course uses Pico2 W/CircuitPython 9.2.8, BME280 at 0x76, and S1; XIAO ESP32-S3 is extra credit. Opening lessons cover Linux/macOS tools, VS Code, wiring, GP14 ROFS/RWFS ownership, CircuitPython installation, serial discovery/run_screen/REPL, deployment, and device identity/IP/hostname discovery before provisioning. Later lessons cover architecture, MQTT, automation, recovery, memory, testing, signed OTA, and an extension project.

Timestamp both successful and failed mqtt send_chunk_slow diagnostics using local date/time or monotonic seconds before clock synchronization. Preserve the existing diagnostic cap and add coverage for valid, unset, and failed clocks. Firmware version is v0.26.257.1.

Related issue

Operator-requested educational documentation and console timestamp improvements; no separate issue.

Verification

• ruff check . — passed.
• pytest -q — 864 passed.
• git diff --check and staged diff check — passed.
• Local links, Python example syntax, and whitespace checked for the root README and all 14 syllabus documents.
• Completed sensor and recovery starter examples — 2 passed; selective REPL config-reader example checked against a host fixture for section/key filtering and exclusion of passwords.
• Hardware lessons, USB flashing/deployment, RF/socket behavior, and broker delivery were not exercised. The runtime change is limited to console timestamp formatting; this PR makes no broker-delivery or on-device validation claim. No mounted device files were written.

Checklist

• I kept the change focused and avoided unrelated reformatting.
• I added or updated focused tests when behavior changed.
• ruff check . passes.
• pytest -q passes.
• I updated docs and TOML templates when public behavior or configuration changed. (No config schema or MQTT payload changes.)
• I did not commit credentials, live device configuration, private logs, or host-specific files.
• I preserved CircuitPython compatibility and constrained-memory behavior.
• I documented relevant hardware validation, or explained why it was not required.
• I updated the firmware version only if runtime behavior changed.