release: add v1.4.0 assets and notes

Author: Vanderhell

releases/v1.4.0/RELEASE_NOTES.md

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+# µMesh v1.4.0
+
+Broader ESP target support release with capability detection and C6 TWT groundwork.
+
+## Added
+- New compile-time target capability header: `include/umesh_caps.h`
+- ESP32 target matrix for ESP32 / S2 / S3 / C3 / C6
+- Unsupported target guards for ESP32-H2 and ESP32-C2
+- Runtime capability API:
+  - `umesh_get_target()`
+  - `umesh_get_wifi_gen()`
+  - `umesh_target_supports(...)`
+- Extended `umesh_info_t` with target metadata (`target`, `wifi_gen`, `tx_power_max`)
+- ESP32-C6 TWT stub (`port/esp32/twt_esp32c6.c`)
+- New capability tests (`tests/test_caps.c`)
+- New target info example (`examples/target_info/`)
+
+## Build/Config
+- Target-aware CMake source selection for ESP32-C6 TWT stub
+- `UMESH_LOW_MEMORY` compile option support
+- Version bump to `v1.4.0`
+
+## Verification
+- Full POSIX test matrix passes with power management ON and OFF.
+
+## Included Assets
+- `umesh-v1.4.0-source.zip`
+- `umesh-v1.4.0-hardware-firmware.zip`
+- `umesh-v1.4.0-target-info-example.zip`
+- `SHA256SUMS.txt`

releases/v1.4.0/SHA256SUMS.txt

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+efcacf9ffe8389470d18179be2d9b44b659871d0085fba7f060630244583194e  RELEASE_NOTES.md
+753448156856ad458491d8ee36d525c741e202f5bed299f650fd44076c1d45a5  umesh-v1.4.0-source.zip
+a56d016a43c24287389c7a7d37db6740ba26666ffed4f458da66a654f5aa16c9  umesh-v1.4.0-hardware-firmware.zip
+a4d2cb77c25eee374eb91cd3604d404480ecf3d081612ff4477662a77064809c  umesh-v1.4.0-target-info-example.zip