[mottihoresh]

Closes #346.

What

The client's npm run sync reads firmware values only from enum [class] X : T { NAME = literal } and constexpr T NAME = literal. Nine protocol facts weren't written that way, so the client hand-typed them. Each one now has a name, and the firmware uses that name in place of the literal.

#	Value	Declaration
1	Gateway is node 0	GATEWAY_NODE_ID (NodeStatus.h). Used at the PanelBridge self-row and at the CANProtocol master checks (NODE_ID != 0, canIdHb(0))
2	_NODE_STATUS row layout	NODE_STATUS_ROW_BYTES (14) + NodeStatusField byte offsets
3	HB flags	enum class HeartbeatFlag { BOFF, EPVF }
4	ESR split	HB_ESR_TEC_SHIFT / HB_ESR_REC_SHIFT
5	Die temp unavailable	DIE_TEMP_UNAVAILABLE = -128
6	_NODE_TELEM framing	NODE_TELEM_ROW_BYTES (9) + NodeTelemField
7	TELEM page layouts	PduRailField / PduStatusField. The PDU packs its frames through them
8	NTC unavailable	NTC_TEMP_UNAVAILABLE = -32768
9	Calibration payload offsets	AxisCal.h: one offset enum for every type that has a payload (HELLO_ACK, CAL_DATA + record, SESSION_OPEN, SESSION_ACK, STREAM_SELECT, COMMIT, RESET, ACK, NACK, RAW) + CAL_DATA_REC_BYTES. SimGateway reads and writes every payload through them

Design choices

• Formatting: the _NODE_STATUS / _NODE_TELEM rows are still built with snprintf. Static asserts tie each format string to its row enum, so moving an enum value fails the build until the format string is updated.
• Sentinels: written as negative literals (-128, -32768), each with a static assert that it equals INT8_MIN / INT16_MIN. One name per sentinel.
• ID, not NODE_ID: the row-field enums name the node-id field ID because every node build defines NODE_ID as a macro.
• Item 9 scope: I added offset enums for the one-byte request payloads and ACK too, beyond the six types the issue lists, so the client needs no hand-typed offset at all.

No wire or behaviour change — proof

I built 20 envs on main (d29433a), then again on this branch, and compared the .bin files: all 20 are byte-identical.

• Envs: E2E SimGateway / PanelBridge / PanelGroup, PDU_Center, Center_Armament, plus the node_status, node_telem, axis_cal_* (4), heartbeat_payload, health_payload, health_flags, node_status_service, pdu_math, board_thermal, rail_monitor, telem_schedule and adc_clock tests.
• I checked first that a clean rebuild of unchanged source reproduces the same binary.
• I also built the first env of every Tests/Panels/Examples project (34/34 OK) to catch macro collisions.

Docs (same PR)

• FirmwarePlan 02 / 03 / 04 and TechSpec CANProtocol / NodeStatus / PanelBridge / PowerMonitor / STM32Board / SimGateway now name the declarations.
• docs/firmware/build-flags.md had two CANProtocol.cpp line citations that this change shifted by two lines. I updated them, and check_doc_refs.py is clean.
• Nothing else in the published docs needs to change: the wire format is the same.

Client follow-up (SkyHawkClient R1)

• constexprNum must accept a leading - for the two sentinels.
• The new enums and constants can be generated from NodeStatus.h and AxisCal.h. I checked that every new declaration matches the current parseEnum / constexprNum regexes. The only exception is the - sign above.