Compile the C ABI in CI, and export the tap::samplerate alias

[tap]

What this changes

Turns SRT_BUILD_CAPI on in the primary CI matrix (Linux and macOS legs), and exports the tap::samplerate CMake alias.

Why

This repo ships a C ABI under tools/capi that the notebooks drive via ctypes, but the option defaults to OFF and CI never turned it on — so nothing in the pipeline compiled it, and a kernel signature change could break the ABI with CI staying green. AmbiTap and DspTap already build theirs.

tap::samplerate is the alias taphouse's namespace convention has documented all along but this repo never created (see tap/TapHouse#6).

Verification

• Configure and build succeed with SRT_WERROR=ON and the new flag.
• libsrt_capi.so links and exports its 8 entry points (nm -D).
• Full suite passes 73/73.

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 exporting nothing — it would pass CI without gating anything. 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. The Windows leg's SRT_WERROR=OFF policy (pending /W4 triage, per docs/PERFORMANCE.md "Known debt") is untouched.
• SampleRateTap::SampleRateTap stays alongside tap::samplerate, so no consumer changes — including RatioTap, which pins this repo as a test-only submodule for its pinned-eps cross-validation.
• Worth flagging: this repo is the family's odd one out twice over on header paths. include/srt/ is an abbreviation matching neither the repo name nor the tap::samplerate namespace, where the convention wants tap/samplerate/. Renaming breaks every consumer #include, so it stays a migrate-when-touched item (tracked in taphouse's README) rather than something to slip into a CI change.
• No contract change, no submodule pin moved, notebooks untouched. The instruction-count baselines are unaffected — this adds a target rather than touching codegen.

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