[codex] Add maintenance timer fixes and traceability notes

[yury-g]

Purpose

This draft PR prepares a maintenance branch for branch-based testing before any merge back to master. It keeps the public version line at 2.5.1 while fixing traceability and timer issues that were found by inspection.

What changed

• Aligns library.properties and in-code version macros with release v2.5.1.
• Adds MAINTAINER_NOTES.md so the maintenance reasoning and follow-up areas are easy to audit later.
• Centralizes sensor-index validation and rejects sensorIndex == SensorCount.
• Makes sawNewSample() report hardware-timer samples as documented.
• Fixes timer maintenance issues found by inspection:
  • nRF52 and Due pause/resume macro mismatches
  • ESP8266 timer setup shadowing/result handling
  • ESP32 timer disable result handling
  • Mega Timer2 disable mismatch
  • Renesas unavailable-timer handling
  • ATtiny 1 MHz missing semicolon
• Adds scripts/end-user-install-test.sh and resources/EndUserInstallTesting.md to test the library as an Arduino IDE ZIP install.
• Adds manual hardware and release handoff docs:
  • resources/ManualHardwareTestPlan.md
  • resources/ReleaseNotes-2.5.2-draft.md
  • resources/MaintenanceBranchHandoff.md

Validation already run

• git diff --check
• XIAO ESP32-S3 direct source-tree compile with arduino-cli
• scripts/end-user-install-test.sh
  • creates a ZIP from the current Git commit
  • installs it into a temporary Arduino sketchbook
  • compiles from the installed library folder
• Remote branch ZIP install test from this PR branch
• Expanded installed-library compile pass on esp32:esp32:XIAO_ESP32S3 for:
  • GettingStartedProject
  • Getting_BPM_to_Monitor
  • PulseSensor_BPM
  • PulseSensor_Pulse_Transit_Time
  • TwoPulseSensors_On_OneArduino
  • PulseSensor_XIAO_ESP32S3_BPM

Known limitation

Local AVR and UNO R4 compiles were attempted, but this Mac's installed Arduino toolchains fail before compilation with bad CPU type in executable. The added end-user install script documents a broader board matrix that can be run on Linux or another machine with compatible Arduino toolchains.

Prepared but not pushed yet

A GitHub Actions Arduino compile matrix exists as local commit 7e5ae73 and as a patch bundle at:

/Users/narwhal2/Documents/PulseSensorPlayground-local-patches/0001-Add-Arduino-compile-smoke-workflow.patch

Pushing .github/workflows/arduino-compile.yml requires the GitHub token to have workflow scope. Once authorized, push the local branch and monitor the checks.

Release intent

After this branch is reviewed, hardware-tested, and merged, prepare a separate 2.5.2 bump branch using resources/ReleaseNotes-2.5.2-draft.md as the release-note starting point.