Story: Clean ores.platform to the component clean standard

ores.platform meets the Component Clean Standard: the items marked All pass, each item that does not apply is recorded with its reason, and ores.platform joins COMPONENTS_UNDER_TEST. Order 1 of 35 in the sprint 26 component clean-up, tier 1 (foundation infrastructure).

• Every checklist item that applies to a component of kind All passes, with its evidence recorded on the task.
• Every item that does not apply is recorded as not applicable, with the reason.
• Every generator defect found is fixed at its source, with a regression test.
• ores.platform is listed in COMPONENTS_UNDER_TEST, and check_component_drift.py --all passes.

• Survey on 2026-09-23: 0 entity, junction or operation models; 32 C++ files outside tests, 0 of them generated (0%); 0 messaging headers, 0 of them hand-written.
• Survey on 2026-09-25, by an agent that did not write the component. 32 production C++ files at baseline in 23 headers and 9 sources, 7 files under `tests/`, 88 test cases and 201 assertions. 458 C++ files outside the component reference `ores.platform/`. Both suites pass. Five public artefacts have no consumer, two MAC enumerators duplicate each other, three of the nine source files have no test, ten test cases are vacuous, and the overview is stale. The full findings and the work list are on the task. The deletions leave 30 production files, 21 headers and 9 sources.
• The diagram cannot be refreshed until its generator is fixed. `build/scripts/generate_component_puml.py` loses brace sync on a one-line `enum class`, so a refresh deletes the `time_unit` members and the whole `relative_time_formatter` class. A dry run reproduces it.
• The component is widely reachable. The largest surface is time, where 383 C++ files outside the component include `ores.platform/time/datetime.hpp`.

• Delete code with no consumer, and code whose only consumer is its own test. H02 forbids dead code, and a test is not a consumer. Recorded as item B06 of the standard.
• Prefer the standard facility over a hand-rolled shim when every supported compiler has one. `ORES_NO_UNIQUE_ADDRESS` gives way to [[no_unique_address]], and `ores::platform::unreachable()` to `std::unreachable()`. Recorded as item H04.
• Fix a generator before refreshing what it generates. A refresh that deletes correct content is worse than a stale diagram. Recorded as item G07.
• A test that passes when its subject does nothing is a finding. Recorded as item V07.
• A source file no test touches is the coverage work item. Recorded as item V08.

• The other 145 stale `component_files.cmake` files. They are one repo-wide sweep, and this story fixes only platform's two.
• The registry gap. platform has no catalogue row and no model, so it cannot join `COMPONENTS_UNDER_TEST`. That needs one programme decision, and six components now share it.
• The generator fix reaches seven components, because seven contain a one-line `enum class` in a header, which is what the parser reads. Two more declare one in a source file the parser never reads. It is fixed here because platform's diagram depends on it, and the regression test goes in the codegen suite.