feat: implement Table 30 Row-2 evt[2:0] validation — ADC (2nd endpoint type)

[SoundMatt]

Direct follow-up to the merged pilot (#114), which established the shared Table 33 Row 2 evt[2:0] classifier (rcp::endpoint::evt_row2_kind_of, EvtRow2Kind{Plain,ConfigWrite,Reserved}) and wired I2C as the first of the 8 Row 2 endpoint types ({ADC, PWM_IN, I2C, LIN, CAN, UART, ISELED, MDIO}) into mock::Server::dispatch(). This PR applies the identical pattern to ADC, the second endpoint type — and, notably, the row's own worked example (TC18 §13.7.9.3 Figure 34, "RC Client sends a standard read request") that the pilot cited to resolve Table 33's evt=000b textual ambiguity in the first place.

What changed

• include/rcp/adc.hpp: adds AdcEndpoint::handle_request(evt_op, cfg, take_sample, out_value), ADC's request-decode entry point, mirroring I2cEndpoint::handle_request's shape exactly.

  • Plain (evt[2:0]==000b) delegates to the existing request_reading() (the SelfTimed cadence pattern — ADC's "standard read request" behavior, matching Figure 34). request_reading_from_trigger_queue's ExternalTrigger cadence remains a separate entry point, not folded into handle_request by this milestone.
  • Reserved returns endpoint::EndpointErrc::reserved_evt_row2 without invoking take_sample or touching out_value.
  • ConfigWrite returns the new AdcErrc::config_write_not_supported rather than crashing or silently accepting/dropping the request — full §12.7.1 config-write handling (EP_functional-config wiring) is deliberately deferred as a follow-up, the same scope decision I2C's own handle_request made, since ADC has no EP_functional-config wiring in this codebase either.
  • Also adds encode_adc_value, a small 16-bit big-endian wire codec for a single measurement value (§13.7.9.3's "16 bit ADC value" field) — needed by the mock dispatch wiring below since ADC previously had no wire codec of any kind (unlike gpio.hpp/pwm.hpp).

• include/rcp/mock.hpp: wires AdcEndpoint into mock::Server::dispatch() at endpoint id/byte_bus_id 4, following the I2C pattern exactly (operational-request gating, wire_error_code_for mapping, a set_adc_response() scripting hook mirroring set_i2c_response() — here scripting a FIFO queue of 16-bit sample values a plain ADC request consumes one of, since ADC's read path needs scripted sample data the same way I2C's did). dispatch_adc() drives handle_request against a default-constructed AdcAveragingConfig (no averaging: one raw sample per request), consistent with ADC having no write-request payload (§13.7.9.3: "The ADC request has no byte_msg_payload"). Register map now describes 4 endpoints instead of 3.

• .fusa-reqs.json: REQ-ADC-007 (encode_adc_value wire codec), REQ-ADC-008 (handle_request Plain/Reserved), REQ-ADC-009 (handle_request ConfigWrite), REQ-MOCK-013 (ADC dispatch wiring + Row 2 classification), REQ-MOCK-014 (ADC operational gating), REQ-MOCK-001 updated for the 4-endpoint map. New entries reuse tc18_master_id TC18-13.5-001 (Table 33's shared Row 2 rule — the same master-catalog fact REQ-ENDPOINT-007/REQ-I2C-006/REQ-MOCK-011 already cite) and TC18-12.7.1-001 (the config-write shape), plus ADC's own §13.7.9.3 Figure 34/35 citations for the wire-codec requirement — verified against this machine's local TC18.txt and RELAY's docs/tc18-master-catalog.json.

Scope decision

Same as the I2C pilot: §12.7.1 configuration-write handling is out of scope here — ADC's evt[2:0]==111b case is recognized-but-deferred (AdcErrc::config_write_not_supported), not silently accepted or ignored. ADC has no EP_functional-config wiring anywhere in this codebase yet.

Sec-tested CI gate

Adding 5 new requirements with none cybersecurity-tagged would have diluted the ratio from 65/381 (17.06%→17%) to 65/386 (16.84%→16%), tripping the >=17% gate. REQ-ADC-008 — the ADC-level "reject a reserved evt[2:0] value with UNSUPPORTED_CMD rather than silently accepting it" property, same class as REQ-ENDPOINT-007's tag in the pilot — is tagged severity: "cybersecurity", restoring the ratio to 66/386 = 17% (integer division, matching CI's own shell arithmetic).

Verification

• Local build (Ninja, C++17, Release): 219/219 targets, 0 new warnings.
• ctest --test-dir build --output-on-failure -j1: 53/53 passed.
• cpfusa v0.18.0 check: 0 errors (2 pre-existing warnings, 1 pre-existing info, unrelated to this change).
• cpfusa v0.18.0 trace --format json: tracedRequirements/testedRequirements = 386/386 (100%); secTestedRequirements/totalRequirements = 66/386 = 17%, meeting the gate.

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