[petesramek]

Description

Adds the Phase 1 adapter contract foundation: all five built-in transport adapters return an allocation-free TinyWriteResult that distinguishes complete, partial, failed, and zero-length requests. Platform write outcomes are inspected and classified; TinyLink core transmission/state propagation is intentionally deferred to layer 2. No issue is linked.

Files changed: CHANGELOG.md, README.md, src/TinyWriteResult.h, src/adapters/TinyArduinoAdapter.h, src/adapters/TinyESPIDFAdapter.h, src/adapters/TinyPosixAdapter.h, src/adapters/TinySTM32HALAdapter.h, src/adapters/TinyWindowsAdapter.h, test/adapters/TinyDuplexTestAdapter.h, test/adapters/TinyTestAdapter.h, test/globals.h, test/test_runner.cpp, test/test_write_result.cpp.

Public API impact, assumptions, and limitations: Adapter write() methods now return TinyWriteResult; existing call sites may ignore the result, but custom adapter overrides must adopt the new return type. bytesWritten reflects the platform-reported accepted count. STM32 HAL exposes all-or-error status rather than a partial count, so its failure result reports zero accepted bytes. Platform SDK builds and hardware tests were not run; native tests validate the shared result model. This layer does not propagate results through TinyLink's core.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactor / code quality improvement
• CI / build / tooling change

Checklist

• I have read the Contributing Guidelines.
• My branch is up-to-date with the target branch.
• My changes compile without errors or warnings on the affected platform(s).
• I have added or updated tests to cover my changes.
• All existing tests pass (pio test -e native).
• I have updated relevant documentation (README, inline comments, etc.).
• My commit messages follow the Conventional Commits format.
• I have added a CHANGELOG.md entry under [Unreleased].

Testing Done

• pio test -e native — PASS, 183 test cases succeeded (native PlatformIO environment, configured for C++11).
• Arduino, ESP-IDF, STM32 HAL, and Win32 SDK-specific builds were not run in this environment.

Screenshots / Logs (if applicable)

Not applicable.