Add I2C target SCL high-spike filtering

[xross]

Summary

• Add a 50 ns SCL high-spike filter for mixed I3C/I2C bus compatibility.
• Add I2C_SLAVE_CLOCK_STRETCH to optionally prevent the target from driving SCL low.
• Add simulator coverage for SCL glitches and non-clock-stretch operation on XS2 and XS3.
• Document both configuration macros.

Testing

• Added dedicated spike-filter and no-clock-stretch tests.

[Copilot]

Pull request overview

This PR adds SCL high-spike filtering to the I2C target (slave) implementation to improve robustness in the presence of short SCL high glitches (e.g., I3C bus coexistence), and introduces simulator-based tests to validate the behavior.

Changes:

• Add optional SCL high-spike filtering logic to i2c_slave.xc and apply it to clock-edge handling and START/STOP detection.
• Add a new Pyxsim/pytest test suite to inject short SCL high glitches at specific bit positions and verify behavior matches the existing expected output.
• Update test dependency wiring, build optimization flags, and the changelog.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
tests/test_slave_scl_spike_filter.py	New simulator test injecting SCL high glitches at various pulse indices.
tests/requirements.txt	Changes how test_support is pulled in for tests (now via relative editable path).
tests/i2c_slave_test/CMakeLists.txt	Changes test app optimization level (-O2 → -O3).
lib_i2c/src/i2c_slave.xc	Implements SCL high-spike filtering and uses it in key state-machine decisions.
lib_i2c/lib_build_info.cmake	Changes library optimization (-Os → -O3) for the CMake-based build path.
CHANGELOG.rst	Adds an UNRELEASED section describing the changes (currently includes an entry not reflected in this diff).

Suppressed comments (1)

lib_i2c/lib_build_info.cmake:13

• This changes the library compiler optimization from -Os to -O3, but the legacy lib_i2c/module_build_info still uses -Os (line 6). That makes optimization level depend on which build system is used; consider keeping -Os here (or updating all build entrypoints consistently).

set(LIB_COMPILER_FLAGS  -O3
                        -Wall
                        -Wextra
                        -Wsign-compare
                        -Wconversion)

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[xross]

Exposed this: #123