Pin M33MU to upstream master with deterministic USART timing

[aidangarske]

• Bump M33MU_REF to f3c03675 (danielinux/m33mu master with # 21) in the M33MU workflow, the STM32H563 runner and the MIMXRT700 runner.
• m33mu # 21 raises the STM32 USART interrupt at the register write instead of the next host poll, and makes --uart-stdout lossless and ordered under a slow host. This removes the per-run interrupt-timing drift behind the occasional confboot test 66 failure and the lost console lines on CI.
• Should close M33MU runs occasionally lose console output or shift interrupt timing on GitHub runners #34

[Copilot]

Copilot review overview

🟡 Changes recommended

Both runners can silently reuse cached binaries built from an older M33MU revision.

Review effort: Balanced
Findings: 2

Open (2)

• Cache reuse ignores pinned source revision · New
• Emulator cache ignores pinned checkout revision · New

What changed in this PR

Pins M33MU to the upstream revision containing deterministic USART timing and lossless console output, while removing the now-upstreamed RT700 patch.

Changes:

• Updates M33MU references across CI and target runners.
• Removes the RT700 emulator patch.
• Updates testing documentation for unpatched upstream M33MU.

File	Description
.github/​workflows/​m33mu.yml	Pins CI to the new revision.
tests/​target/​run_m33mu_scenario.sh	Updates the STM32H563 runner pin.
tests/​target/​run_rt700_m33mu.sh	Updates the RT700 pin and removes patch application.
tests/​target/​m33mu-imxrt700.patch	Deletes the upstreamed patch.
docs/​Testing.md	Documents using unpatched upstream M33MU.

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