Fix v0.4.0 mtmd helper compatibility

[leehack]

Summary

The automatic llama.cpp v0.4.0 release failed compiling the TTS speaker-reference path: mtmd_helper_bitmap_init_from_buf now requires helper options. Detect that API at configure time and pass upstream defaults when available, retaining the four-argument path for the currently pinned v0.3.0.

Add a model-free, real-image decode test and run the complete wrapper contract against both the pinned revision and immutable v0.4.0 commit 5266f24da75dc449bd56cbed7addb9c8e4a6a73e in PR CI. The original wrapper-contract check name is retained.

Closes #71.

Validation

• macOS arm64 CPU Debug builds against v0.3.0 and v0.4.0: PASS, including the wrapper and TTS smoke executable.
• CTest on each revision: 3/3 PASS (speculative API, TTS API, real media compatibility); 36/36 required exports PASS.
• Real Qwen3-TTS 1.7B Base Q4_K_M + Q8_0 projector, with nonempty speaker-reference audio: PASS on both revisions using tools/tts_smoke.cpp. Cancellation/reset, two sequence syntheses, finite/non-silent 24 kHz PCM, and WAV output passed. Output WAVs were byte-identical: SHA-256 16dc619f392f905d32b9b03c588ba4d424218ff9830096207bb99673273c6cf9.
• Model SHA-256 8d18c94acb2addd042f97da63c98be144eafa76d0d9495177eab65130cf85129; projector 6fd65188839bcd6ecc91b277ad471e22a0edfada4699a0fe82f1165c18cfcce2; speaker WAV de964f9d761766be44fdf648c961f9232cd53cb14373c20b6046106f06661203. Text Hi, language en, seed 1, Apple M4 Max CPU. This proves speaker-reference integration, not voice-fidelity or hardware-GPU qualification.
• Actionlint, release provenance verifier, and git diff --check: PASS.
• Independent Sol review: PASS on commit 80278f2c8cb96bb0080be16a95081323dee5c6b6; no remaining blocker. Reviewer independently reran the test executables and export checks for both revisions.
• Exact-head hosted CI: 7/7 PASS: pinned and v0.4.0 wrapper contracts, Linux x64/arm64 artifact contracts, Windows MSVC link contract, Python 3.10 artifact tools, and release provenance verification. Wrapper matrix, provenance/tools.
• Final review-thread audit: 0 total / 0 unresolved; GitHub MERGEABLE/CLEAN. No GitHub review is populated; independent Sol review is recorded above. Zero known PR-caused regressions remain.

The exported wrapper ABI, submodule pin, release policy, and downstream pins are unchanged. A complete successful release matrix is still required before any v0.4.0 artifact is published. The equivalent Web call-site prerequisite is tracked separately in leehack/llama-web-bridge#90.