S08.03 slice 6: cmdline -> set translation in target_driver (untag prival, message_fields, udp_mtu)

[DavidCozens]

Parent epic: #268 (S08.03 — UDP syslog from FreeRTOS reaches the oracle)

Background

S08.03 slice 5 (#305) landed the FreeRTOS BDD harness and tagged eight @udp features with @freertoswip so the new bdd-freertos-qemu job could go green on a single passing scenario. Slice 4 (#303) shipped the set NAME VALUE interactive command channel that mutates the FreeRTOS example's identity, transport endpoint, and PRIVAL fields in-RAM. The two pieces compose: the BDD driver can now drive the FreeRTOS example into any configuration the existing Linux scenarios need, but the bridge — translating --flag VALUE cmdline pairs into set NAME VALUE UART writes — does not yet exist. Today target_driver.spawn_example_process raises NotImplementedError whenever extra_args is non-empty under BDD_TARGET=freertos.

Scope

Add the cmdline -> set translation in the FreeRTOS branch of the BDD target driver, loosen the FreeRTOS example's OnSet facility/severity input validation to match Linux's permissive atoi-and-cast, and untag every @udp scenario that the combination unblocks.

Translation table (exactly four entries; no premature generalisation):

Cmdline flag	FreeRTOS interactive command
--facility N	set facility N
--severity N	set severity N
--msgid X	set msgid X
--message X	set msg X

These are all the flags that @udp features currently pass to run_example. Any other flag must raise from the driver so a misconfigured scenario fails loudly with a clear Python traceback rather than silently no-op'ing or hitting a confusing set rejection.

Design

• Spawn (target_driver.spawn_example_process): on FreeRTOS, no longer raises on non-empty extra_args; QEMU argv is unchanged.
• New helper target_driver.apply_extra_args(context, process, extra_args) called by _run_with_prompt_protocol immediately after wait_for_prompt(process). On Linux/Windows it is a no-op (flags already in argv from spawn). On FreeRTOS it walks the extra_args list pairwise, looks each --flag up in the translation table, and writes one set NAME VALUE\n line per pair to process.stdin. An unknown flag, an odd-length args list, or a value missing from the table raises with the offending flag in the message.
• Example/FreeRtos/SingleTask/main.c::OnSet: drop the parsed > 23U (facility) and parsed > 7U (severity) upper-bound rejections. This matches Linux's atoi-and-cast — the example becomes permissive about the value and the library is the single authority on what's valid (the library encodes out-of-range facility/severity as the internal-error PRIVAL 43).
• The cmdline --message ↔ FreeRTOS set msg naming gap stays in the table; the example global is g_msg (Example/FreeRtos/SingleTask/main.c#L100) and renaming the FreeRTOS-side command would churn the example for cosmetic gain.

Two-commit plan (single PR, single issue)

Strict TDD: untag, run red, write minimum translation, run green, commit. Repeat.

Commit A — feat: S08.03 slice 6a apply_extra_args helper + facility/severity translation

• New apply_extra_args helper covering only --facility / --severity.
• _run_with_prompt_protocol call site.
• target_driver.spawn_example_process no longer raises on FreeRTOS extra_args.
• OnSet loosened for facility/severity (drop upper-bound rejection).
• Untag prival.feature (drop the feature-level @freertoswip). All six scenarios — including the two Out-of-range scenarios — should pass against syslog-ng-freertos.

Commit B — feat: S08.03 slice 6b msgid/msg translation, untag message_fields + udp_mtu

• Extend translation table with --msgid and --message.
• Untag message_fields.feature::Message ID appears in the message and ::Message body appears in the message. The third scenario (Complete RFC 5424 message with all fields) stays tagged — it asserts system hostname / current timestamp / process PID, which slice 6 alone cannot satisfy.
• Untag udp_mtu.feature::Full delivery of a UTF-8 message within the path MTU. The oversize scenario stays @windows_wip (orthogonal to FreeRTOS).

Out of scope

• syslog.feature (the single scenario), header_fields.feature::Hostname and ::Process ID, and message_fields.feature::Complete RFC 5424 message: assert system identity / current time. These cannot be unblocked by slice 6 alone — they need a follow-up to either drive set hostname / set procid from the BDD step (and a real clock for timestamp) or loosen the assertions on FreeRTOS targets. Tracked under epic S08.03: UDP syslog from FreeRTOS reaches the oracle #268.
• Meta / origin / timeQuality SD wiring in the FreeRTOS example. Tracked under epic S08.03: UDP syslog from FreeRTOS reaches the oracle #268 as separate slices.
• Real RTC clock callback. Tracked under epic S08.03: UDP syslog from FreeRTOS reaches the oracle #268.

Acceptance

• behave-freertos against syslog-ng-freertos runs prival.feature (6 scenarios), message_fields.feature::Message ID, ::Message body, and udp_mtu.feature::Full delivery within path MTU all green; the residual @freertoswip count drops by these scenarios.
• behave-linux regression: full Linux suite remains green (no behaviour change on Linux/Windows targets).
• Pre-PR Linux-side checks pass: build-linux-gcc, build-linux-clang, sanitize-linux-gcc, coverage-linux-gcc, analyze-tidy, analyze-cppcheck, analyze-format.
• DEVLOG appended.

References

• DEVLOG 2026-05-09 slice 5 entry — full per-feature @freertoswip rationale and the originally-suggested follow-up ordering.
• Bdd/features/steps/target_driver.py — current NotImplementedError raise site.
• Bdd/features/steps/syslog_steps.py::_run_with_prompt_protocol — call-site for apply_extra_args.
• Example/FreeRtos/SingleTask/main.c::OnSet — FreeRTOS-side set handler (target of the translated commands; the upper-bound loosening is the only C-side touch).