chore(ch32v): tighten toolchain and build hygiene

[zackees]

Closes #1129

Pins SHA-256 checksums for all four xPack RISC-V toolchain assets using the release's published .sha sidecars; rejects malformed non-CH32V MCU names instead of silently selecting the V003 fallback; documents V30x codegen divergence and known pin-map reuse; and adds CH32V/WCH-LinkE USB CDC control-line guidance.

The deploy-message items in the issue refer to a minichlink fallback path that is not present in the current daemon implementation, so no unrelated fallback code was invented.

Validation:

• soldr cargo fmt --all
• soldr cargo test -p fbuild-toolchain toolchain::riscv --lib
• soldr cargo test -p fbuild-build-mcu ch32v::orchestrator --lib
• soldr cargo check -p fbuild-toolchain -p fbuild-build-mcu
• git diff --check

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📥 Commits

Reviewing files that changed from the base of the PR and between 44c0be3 and 203b318.

📒 Files selected for processing (4)

• crates/fbuild-build-mcu/src/ch32v/README.md
• crates/fbuild-build-mcu/src/ch32v/orchestrator.rs
• crates/fbuild-toolchain/src/toolchain/riscv.rs
• docs/usb-cdc-control-line-matrix.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch chore/issue-1129-ch32v-hygiene

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