perf(comdelay): DIVIDER=0 passes comStatus straight through (TX-done-paced downlink)

[Mikefly123]

Problem

File downlink over the USP LoRa radio is throughput-capped by ComDelay: comStatusIn_handler only latches m_last_status, and run_handler (10 Hz via rateGroup10Hz -> downlinkDelay.run, topology.fpp:233) releases at most one comStatus per tick even when DIVIDER=0 — a ~10 frames/s ceiling (~2 kB/s at 248 B frames) regardless of air rate.

Fix

DIVIDER == 0 -> forward comStatusIn to comStatusOut immediately (TX-done-paced, no rate-group quantization). DIVIDER > 0 -> existing latched/tick-paced behavior unchanged.

Coherence: in passthrough mode the valid flag is never set, so a status cannot also be emitted by run_handler (no double emit). A status latched under DIVIDER > 0 before a runtime switch to 0 is still consumed by run_handler's compare_exchange (no loss). ComDelay is passive, so the forward executes on the radio-side caller thread — safe because comStatusOut feeds ComQueue's async input (enqueue only). A grep-able image-currency marker string is included (comdelay-div0-passthrough-20260723).

No existing ComDelay UT harness in the repo (CMakeLists UT block is the commented template), so no UT was added per scope; behavior was bench-validated instead.

HWIL A/B (2026-07-23, v5e flight + GRC-USP ground, P4_GFSK_75K both ends)

Radio-only downlink path (downlinkRepeater.CHANNEL_ENABLED=[DISABLED,ENABLED,ENABLED], downlinkDelay.DIVIDER=0), 178,704 B file via FileDownlink.SendFile, timed SendStarted->FileSent; flashed image currency verified by reading the marker string back from board flash (0x100b3b87).

Run	Image	Duration	Effective B/s
baseline	latched (fork b7931b1)	90.19 s	1981
passthrough #1	this PR	69.75 s	2562
passthrough #2	this PR	71.47 s	2500

~1.28x speedup; the link is now air-rate/TX-done-limited instead of tick-limited. Received files byte-identical to baseline (identical cksum). No QueueOverflow storms (baseline itself showed one benign ComCcsdsLora QueueOverflow; passthrough runs showed none), no panics/wedges/reboots. Cycle-slip warnings rose 42 -> ~200 per run (warnings only, known under RF+TM load). Regression checks on the passthrough image: DIVIDER=9 still tick-paces (~0.8 frame/s observed on a 3 kB partial), profile sweep P1->P2->P3->P4 + NO_OP all green.

🤖 Generated with Claude Code

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Stack note (2026-07-30): rebased onto post-#477 feat/usp-radio (patch-collapse; now at c6df588), resolving the stale base. This PR is now the bottom of a native GitHub stack: feat/usp-radio <- #475 <- #479. The passthrough unit-test coverage lives in #479 (which stacks on this branch and ports the passthrough into ComDelayLogic).

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