Send 'Sequence completed in N ms' message at trial end

[mbreiser]

Summary

• The host wrapper PanelsController.trialParams() with waitForEnd=true waits for an end-of-trial response whose payload matches \"Sequence completed in N ms\". The previous empty-string response caused that path to time out and return false even when the trial ran successfully.
• This change fills in the formatted message at both end-of-trial sites in serviceDisplay() (PLAYING_PATTERN and ANALOG_CLOSED_LOOP), using a 40-byte stack buffer and snprintf.
• runtime_duration_ms_ is uint32_t (CommandProcessor.h:51); cast to unsigned long for %lu portability.

Why

On a fresh bring-up of a 2×12 arena from macOS using maDisplayTools examples/oldButFunctional/G41_Modes_Demo.m, both mode 2 and mode 3 trials played correctly on the panels but trialParams() returned false. Tracing showed the host's expectResponse inner call (PanelsController.m:631) waiting up to deciSec/10 + 1 seconds for the formatted completion string and never matching against the empty payload.

Compatibility

• The YAML/CommandExecutor experiment-runner path uses trialParams(..., waitForEnd=false), which only consumes the immediate ACK and never reads the end-of-trial message (CommandExecutor.m:139,153,168). That path is unchanged by this PR — we still send a TRIAL_PARAMS_CMD response, just with non-empty payload.
• No new include needed; snprintf is already used in NetworkManager.cpp:24,28.

Test plan

• Verified on hardware: rebuilt with pio run -e teensy41-printf -t upload, uploaded to a Teensy 4.1 connected to a 2×12 arena, ran examples/oldButFunctional/G41_Modes_Demo.m mode 2 and mode 3 against IP 10.102.40.124.
• Both trials play visibly and Result: true returns within the expected duration.
• (Reviewer) Cross-check that no expectResponse consumer is sensitive to the longer payload size (29-byte string + 3-byte header = 32 bytes vs 3 bytes previously).

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