Add an ArduinoJSON arena defaulting to internal memory

[kahrendt]

This avoids heap allocation churn while also saving a few CPU cycles by avoiding PSRAM/reducing cache pressure for these routine messages.

Defaults to a 2048 byte arena in internal memory. ArduinoJSON uses 1024 of those bytes for small messages, and the rest is a copy of the original message.

[Copilot]

Pull request overview

This PR introduces a reusable ArduinoJson allocator backed by a fixed internal-RAM bump arena (with PSRAM/heap fallback) and wires it into SendspinClient::process_json_message() to reduce allocation churn and PSRAM traffic on the network thread.

Changes:

• Added SendspinArenaAllocator (bounded bump allocator with fallback) and make_json_document(arena) helper.
• Added SendspinClientConfig::json_arena_size (default 2048 bytes) and a SendspinClient-owned arena reused across incoming JSON messages.
• Documented the new arena behavior in internal and integration docs.

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
src/platform/json_arena.h	New internal-RAM bump arena ArduinoJson allocator with PSRAM-preferring fallback.
src/client.cpp	Creates optional arena from config and reuses it for parsing incoming JSON messages.
include/sendspin/config.h	Adds json_arena_size config knob with default and documentation.
include/sendspin/client.h	Adds forward declaration and member for the arena allocator.
docs/internals.md	Documents incoming JSON parse arena and updates references to client.cpp.
docs/integration-guide.md	Adds json_arena_size to the configuration table.

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