Compile the C ABI in CI so the verification layer cannot rot

[tap]

What this changes

Turns TAP_RATIO_BUILD_CAPI on in the primary CI matrix (Linux and macOS legs).

Why

This repo ships a C ABI under tools/capi that the executed notebook drives via ctypes, but the option defaults to OFF and CI never turned it on — so nothing in the pipeline compiled it. A change to a kernel signature could break the ABI, and the notebook with it, while CI stayed green. AmbiTap and DspTap already build theirs in CI; this brings RatioTap in line.

Verification

• Configure and build succeed with TAP_RATIO_WERROR=ON and the new flag.
• libratio_capi.so links and exports its 11 ratio_* entry points (nm -D).
• Full suite passes 58/58.

Notes for the reviewer

• Enabled on Linux and macOS only. tools/capi carries no __declspec(dllexport) (unlike DspTap's), so an MSVC build would link a DLL that exports nothing — it would pass CI without gating anything, which is worse than not running it, because it reads as coverage. Recorded in the matrix comment and in taphouse's known-divergences list, to be flipped on in the same change that gives the C ABI an export decoration.
• This repo is one of only two libraries that already fully complied with taphouse's namespace convention on all three counts (tap::ratio namespace, tap::ratio alias, include/tap/ratio/ headers) — it was simply missing from the table. Record the namespace convention's real state, and the family's open divergences TapHouse#6 adds it.
• No contract change, no submodule pin moved, and the notebook is untouched, so its committed executed outputs remain valid. The instruction-count baselines in bench/baselines.json are unaffected — this change adds a target, it does not touch codegen.

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