docs(ch32v): console-output decision + verified wlink probe operations (#1208)

[zackees]

Closes the last software-reachable Phase 3 item from #1208 — "decide and document the SDI-print / serial story" — and corrects probe guidance against wlink v0.1.2's actual behaviour.

Console output on a part with no USB

CH32V003 has no USB peripheral, so there is no target-side CDC port for fbuild monitor to open. Two real options, documented with a recommendation rather than a survey:

SDI-print over the debug link (recommended) — wlink sdi-print enable. Carries a virtual serial channel over the same single-wire SWIO connection used for flashing, so it costs no extra pin and no USART peripheral. That matters concretely: Phase 1 measured a bare blink at RAM 1180 / 2048 B, leaving ~868 bytes, and a USART driver plus buffers is real budget against that.

Caveats that keep it from being a drop-in, all documented:

• enable implies --no-detach, so it holds the chip attached and conflicts with flashing on the same probe — print and flash must be sequenced.
• It is a debug-module channel, not a UART: no baud, no DTR/RTS, so docs/usb-cdc-control-line-matrix.md does not govern it.
• It is not wired into fbuild monitor. Integrating it would mean teaching the serial layer about a non-serial transport — larger than this bring-up.

USART to an external bridge — the fallback, for when SDI-print's attach behaviour gets in the way or the console must survive without the probe.

Probe operations — each verified against installed wlink v0.1.2

Software mode switch. wlink mode-switch --rv works without touching the probe. The README I wrote in #1210 described button-hold as the only route; it is really the fallback for when wlink cannot reach the probe yet (wrong mode + no driver bound is the chicken-and-egg). Corrected.

Target power. wlink set-power enable3v3 powers the target off the probe — no external supply needed for a bare V003.

list is a false-negative trap. With no probe attached:

Command	Exit	Output
wlink status	1	Error: WCH-Link not found, please check your connection
wlink flash <file>	1	Error: WCH-Link not found, please check your connection
wlink list	0	(nothing)

A presence check built on list would read exit 0 as success. Presence must key off status. The Phase 0 test already uses status; this documents why so it doesn't get "simplified" later.

The non-zero exit on flash is also the reason WlinkDeployer's result.success() is trustworthy — there is no path where a missing probe is reported as a successful flash.

Test plan

• soldr cargo test -p fbuild-deploy --lib wlink:: → 1 passed, 3 ignored (hardware/network-gated), 0 failed
• soldr cargo clippy -p fbuild-deploy --all-targets -- -D warnings → clean
• soldr cargo fmt --all
• Every wlink behaviour claim above was executed against the installed v0.1.2 binary, not read from docs

Docs-only apart from the test doc-comment. Refs #1208.

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• crates/fbuild-build-mcu/src/ch32v/README.md
• crates/fbuild-deploy/src/wlink.rs

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