dev-qemu: select MCTP medium via sp-serial feature

[dymk]

Adds an off-by-default sp-serial cargo feature to dev-qemu.

• Default build keeps SmbusEspiMedium (default_smbusespi), so ec-test-cli and the in-repo integration-test job keep working.
• sp-serial selects MctpSerialMedium via uart_service::default_mctp_serial for the two-QEMU SP↔EC serial link. Both arms are one-liners over uart-service's per-medium constructors — dev-qemu holds no wire addressing and needs no direct mctp-rs dep.
• Enabling serial pulls in crc + crc-catalog (DSP0253 FCS-16); cargo-vet exemptions added.

Depends on the uart_service::default_mctp_serial constructor (dymk/embedded-services#12).

TEMPORARY [patch]: pins embedded-services crates to a dymk fork commit = the EC's current pin (62d4ea9) + only the default_mctp_serial constructor, so this builds with zero platform-common churn. Remove once the constructor merges upstream and the EC is modernized to embedded-services main (tracked as a separate epic — the EC is currently 8 commits behind, and bumping requires a battery-service Registration-model migration).

Verification: all build/lint/supply-chain gates green in both feature configs (fmt, clippy ×2, build ×2, check --all-features, feature-powerset ×4, machete, deny). The two-QEMU SP↔EC e2e (test-sp-ec-link) is pending devcontainer verification. (cargo +1.83 MSRV and the local cargo-vet store-format error are pre-existing on main, unrelated to this change.)

Fork PR for review before promoting upstream.

Assisted-by: GitHub Copilot:claude-opus-4.8

[dymk]

E2E verified green. The serial change was proven end-to-end on the two-QEMU SP↔EC harness.

Because EC OpenDevicePartnership#28 (current main) migrated dev-qemu off the QEMU virt machine onto a custom ec machine that the qemu-sbsa harness doesn't yet launch, the current-main EC can't be exercised on the harness directly (orthogonal blocker, tracked separately). So this exact serial change was cherry-picked onto the last virt-based main (f311134, branch dymk/verify-serial-on-virt = eba2ac6), built --features sp-serial, and run through make -C e2e-tests test-sp-ec-link:

=== Running two-QEMU Thermal e2e ===
[PASS] ffa_version
[PASS] ffa_id_get
[PASS] partition_discovery
[PASS] thermal_get_temperature
RESULT: ALL TESTS PASSED

The EC-origin DeciKelvin round-tripped through the real SP↔EC MCTP-over-serial link — confirming default_mctp_serial / MctpSerialMedium is wired correctly.