Cut stack use on the JSON receive path

[kahrendt]

The ESP httpd task runs with the ESP-IDF default 4 KB stack (HTTPD_DEFAULT_CONFIG), and every inbound JSON message is parsed and dispatched on it, underneath httpd's own call chain. Measured with -fstack-usage on xtensa-esp32s3 g++ 14.2 at -Os, a server/state message carrying metadata cost about 1250 bytes of library frames alone, before httpd's own usage below it and before any logging on top. That is a third of the whole stack spent in this library, and it is what the reported httpd stack overflow (issue #102) was eating into.

Almost all of it was avoidable duplication rather than real working set:

• server/state was parsed into one ServerStateMessage aggregate (288 bytes), so every section's storage stayed live in the caller's frame for the whole parse while each section parser built a second copy of the same fields in its own. A ServerMetadataStateDelta is 200 bytes and was materialized twice at once, then a third time by the by-value handle_server_state parameter.

• GCC inlined the section parsers into process_server_state_message and did not overlap their locals, so metadata, color, and controller storage all coexisted in a single 736 byte frame, the largest in the library.

server/state is now parsed one section at a time. Each section is an out-of-line function that fills a caller-owned struct in place, the client scopes each section separately so the compiler can reuse the slots, and a section is only parsed when its role is present. The three handle_server_state overloads take an rvalue reference instead of a by-value copy. stream/start gets the same in-place treatment for its player, artwork, and visualizer sections.

Measured frames, before -> after:

process_json_message              512 -> 320
process_server_state_message      736 -> gone
  process_server_state_metadata          288
  process_server_state_controller        224
  process_server_state_color             192
process_stream_start_message      496 -> 448

Deepest server/state path: 1248 -> 608 bytes (51% less). Deepest
stream/start path: 1008 -> 768.

No behavior change: same fields, same validation, same tri-state delta semantics. tests/test_protocol.cpp moves to the per-section entry points.

Closes #102. It seems like this is a healthy enough margin without needing to increase the default task stack. The encryption work will require a small bump to the httpd task stack.

[Copilot]

Pull request overview

Reduces ESP HTTP task stack usage by parsing JSON role sections independently and in place.

Changes:

• Splits server/state parsing into per-role functions.
• Eliminates intermediate copies through rvalue-reference handlers.
• Updates stream/start parsing and protocol tests.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/client.cpp	Parses and dispatches state sections individually.
src/protocol.cpp	Implements in-place section parsing.
src/protocol_messages.h	Replaces aggregate parser API.
src/metadata_role.cpp	Accepts metadata deltas by rvalue reference.
src/metadata_role_impl.h	Updates metadata handler declaration.
src/controller_role.cpp	Accepts controller state by rvalue reference.
src/controller_role_impl.h	Updates controller handler declaration.
src/color_role.cpp	Accepts color deltas by rvalue reference.
src/color_role_impl.h	Updates color handler declaration.
tests/test_protocol.cpp	Tests the new per-section parsers.

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