Spec-conformant wire: A.1.6 checksum, A.1.5 reply layout, ISDU FlowCTRL/CHKPDU, indices, timing

[w1ne]

Fixes #23 (device side).

Breaking wire change: the device now speaks the IO-Link V1.1.5 wire byte for byte. Nothing built against the previous checksum or ISDU framing can talk to it.

• Message checksum per A.1.6: XOR with seed 0x52, compressed 8 to 6 bits. Test vectors derived from the spec formula and cross-checked against lwIOLink.
• Reply layout per A.1.5: [PD-in][OD] CKS, Event flag bit 7, PD-invalid bit 6. No leading status octet.
• OD dispatch by communication channel (Page, Diagnosis, ISDU); CKT type bits honoured; illegal M-sequence type returns to STARTUP.
• ISDU transport per 7.3.6 / A.5: FlowCTRL in the MC address (START, COUNT, IDLE, ABORT), Length/ExtLength counting every octet, CHKPDU, Busy and No Service octets, Annex C error types. Byte-exact tests for the Figure A.20 examples.
• Diagnosis channel serves the Table 58 event memory; StatusCode write acknowledges events; SystemCommand is write-only.
• Standard index map per Table B.8 (0x0020 ErrorCount, 0x0024 DeviceStatus, 0x0025 DetailedDeviceStatus, vendor objects at 0x0040+); DeviceID is 3 octets; M-seqCapability codes per Table A.10.
• Timing: single T_REN (500 us), T_WU, T_DSIO, T_FBD.
• Python virtual master and the conformance suite drive the new wire.

Verification: 22/22 ctest, check_quality.sh clean, 14 virtual-master unit tests, conformance suite 48/49 (the remaining failure, test_04_pd_exchange_consistency, is host scheduling jitter and fails on unmodified develop too).

Design and plan: docs/superpowers/specs/2026-09-18-spec-conformant-wire-design.md, docs/superpowers/plans/2026-09-18-spec-conformant-wire-device.md.