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fix: size bb_pub worker stack for the serialize path + move buffer entry off-stack (B1-349) (#527)

Plain-MQTT boards got CONFIG_BB_PUB_WORKER_STACK=4096, sized only for the sink
publish. But the worker also serializes every source each tick — cJSON +
newlib dtoa for float fields (e.g. soc_c) is a deep, ~2KB heap-allocating stack
chain. On 4096 a float-bearing payload overflowed the worker stack into the
heap, corrupting metadata and crashing a later alloc (seen in bb_pub_tick_once
-> bb_json_serialize -> dtoa -> calloc, and bb_ring_push). Raise the non-TLS
default to 6144. Also make buffer_capture's BB_PUB_BUFFER_ENTRY_MAX scratch
static (off-stack) to cut peak stack further. Diagnosed from tdongle coredumps.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Claude-Session: https://claude.ai/code/session_01Ly8KysyxqixYGEbgtabjMN

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