fix(ci): fix math / st7123touch / meshtastic example builds on ESP-IDF v6

[finger563]

After the CI bump to ESP-IDF v6 (#751), three example builds fail. v6 ships GCC 15.2 and compiles IDF’s own components with -Wall -Wextra -Werror, so warnings the older toolchain tolerated now break the build.

Fixes

• math (esp32): the example applied -Wdouble-promotion project-wide (add_compile_options before project()), so it was also compiled into ESP-IDF — whose esp32 code (esp_hw_support/port/esp32/rtc_clk.c) trips it under the new GCC and fails IDF’s -Werror build. Scope the flag to the example’s own translation units via target_compile_options(${COMPONENT_LIB} PRIVATE -Wdouble-promotion) in main/CMakeLists.txt, keeping the float-only check without touching IDF. (The example is deliberately float-pure — all 0.05f literals — so it still passes.)
• st7123touch (esp32s3) / meshtastic (esp32s3): -Werror=missing-field-initializers flagged the std::function Config members with no default member initializer (St7123Touch::Config::write/read, MeshtasticNode::Config::transmit) when a designated-initializer aggregate omits them. Give them a default {nullptr}, matching their sibling members (e.g. meshtastic’s on_text{nullptr}) — fixing it for every caller, not just these examples.

Verification (IDF v6.0.1 / GCC 15.2, local)

• math (esp32) → Project build complete
• st7123touch (esp32s3) → Project build complete
• meshtastic (esp32s3): transmit was its sole missing-initializer error (identical pattern to st7123touch, which builds clean); it pulls in the large BSP/lvgl submodules, so it was not rebuilt locally.

🤖 Generated with Claude Code

[Copilot]

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Pull request overview

Fixes ESP-IDF v6 (GCC 15.2, -Wall -Wextra -Werror) example build failures by scoping a warning flag to the example component and providing default initializers for omitted std::function/callable config members.

Changes:

• Scope -Wdouble-promotion to the math example component via target_compile_options() instead of applying it project-wide.
• Add default {nullptr} initializers for config callables in st7123touch and meshtastic to avoid -Wmissing-field-initializers becoming an error under -Werror.
• Document why the warning flag must not be applied globally in the math example’s top-level CMakeLists.txt.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
components/st7123touch/include/st7123touch.hpp	Adds default initialization for config callable members to avoid missing-field-initializers errors.
components/meshtastic/include/meshtastic.hpp	Adds default initialization for the transmit callback to avoid missing-field-initializers errors.
components/math/example/main/CMakeLists.txt	Applies -Wdouble-promotion only to the example component target.
components/math/example/CMakeLists.txt	Removes project-wide warning flag application and documents the rationale.

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[github-actions]

✅Static analysis result - no issues found! ✅

[finger563]

Follow-up: meshtastic had a second v6 break beyond the transmit initializer (the build stops at the first error, so it was hidden). Now fully built locally.

• mbedtls 4.x (fb0475655): ESP-IDF v6 ships mbedtls 4.x, which removed the public <mbedtls/aes.h> (the low-level AES API moved to <mbedtls/private/aes.h> as PSA Crypto became the public interface). meshtastic_crypto.cpp uses mbedtls_aes_crypt_ctr for the Meshtastic AES-256-CTR cipher, so it failed with mbedtls/aes.h: No such file. Selected the header by MBEDTLS_VERSION_MAJOR so it builds on both mbedtls 3.x (IDF v5) and 4.x (IDF v6); the mbedtls_aes_* calls are unchanged.

meshtastic (esp32s3) now builds clean on IDF v6.0.1 — so all three examples in this PR are verified end-to-end.