health: a CLI dump of where the main loop's time goes

Every probe of a misbehaving switch so far measured it from the
outside. This puts the witnesses inside.

Each idle() pass increments a loop counter and stamps a checkpoint
after every phase: link polling, SFP, RX, TX, STP and the command
dispatch. A phase spanning two or more system ticks counts as slow and
its worst time is kept, in 5 ms units. handle_rx counts frames, so the
dump separates a flooded CPU from a stalled loop.

At boot the free stack area is painted, preserving the interrupt state
around the painting, and the dump reports how much of the pattern
survives next to the live stack pointer. The TCP connection table is
printed whole: state, timers, retransmissions, ports and peer, next to
the httpd transfer state, so a permanently occupied slot names its
occupant. With more than one slot configured every slot is listed.

Everything is raw hex; two dumps a known time apart give the rates.
The implementation sits in cmd_parser next to the other command
handlers and needs no storage beyond its counters.

cmd_parser.h

@@ -9,6 +9,22 @@ extern __xdata uint8_t cmd_buffer[CMD_BUF_SIZE];
 extern __xdata uint8_t cmd_available;
 extern __xdata uint8_t err_status;
 
+/* Phase ids of the health instrumentation, in main-loop order */
+#define HEALTH_PH_LINK	0
+#define HEALTH_PH_SFP	1
+#define HEALTH_PH_RX	2
+#define HEALTH_PH_TX	3
+#define HEALTH_PH_STP	4
+#define HEALTH_PH_CMD	5
+#define HEALTH_PHASES	6
+
+extern __xdata uint16_t health_rx_frames;
+
+void health_loop_start(void) __banked;	/* once per idle() pass, resets the phase baseline */
+void health_phase(uint8_t id) __banked;	/* after each phase; accounts ticks since last checkpoint */
+void health_stack_paint(void) __banked;	/* paint the unused stack area once at boot */
+void health_show(void) __banked;	/* "health" CLI dump */
+
 void cmd_tokenize(void) __banked;
 void cmd_parser(void) __banked;
 void execute_config(void) __banked;