media: harden ordered image, audio, MP3, and WebP handling

[pd95]

Motivation

Media requests can expand substantially while being decoded or converted.
Without explicit bounds, one request can retain excessive decoded audio,
oversized WebP intermediates, or too many individually valid media items before
model execution begins. Format handling was also inconsistent between the CLI,
Gemma, MLX, and llama.cpp request paths.

These are shared request-boundary and interoperability concerns rather than a
single model's implementation details.

What this changes

• adds MP3 decoding through a small miniaudio wrapper with explicit
  input/output limits and a clear no-CGO rejection path;
• accepts MP3 attachments consistently through the CLI and Gemma 4 path;
• bounds WebP dimensions and decoded allocation before conversion;
• applies WebP normalization consistently to llama.cpp completion media;
• enforces cumulative request-wide limits across ordered image and audio
  inputs, not only a limit on each attachment;
• adds malformed and boundary coverage for decoders and request routes;
• documents MP3 CLI support and the vendored decoder dependency.

Replacement, dependency, and scope

Official Ollama v0.34.0 contains the core Gemma 4 image/audio implementation
previously proposed in closed PR #22. This PR supersedes #22, is based on
Apertus PR #6, and carries only the remaining shared-media work. It preserves
upstream's bounded chunking and 600-second audio cap.

Compare:
feature/apertus-mlx...feature/gemma4-mlx-multi-media

Validation

CGO-enabled media, CLI, file-data, LLM, and Gemma tests pass, as does the
explicit no-CGO MP3 rejection path. Required Linux lint/build and serialized
Darwin MLX build gates also pass. The vendored miniaudio.h whitespace is the
only approved diff-hygiene exception.

Packaged ordered-media runtime validation remains part of the release gate.

Refresh and AI assistance

Adapted onto Ollama v0.34.0; head
c17a130842ad9555350e8c49a621776394771a37. Codex materially assisted
implementation review, tests, refresh validation, and this PR description.

[pd95]

This PR replaces the earlier v0.32.x review in #17 after the stack was refreshed onto the v0.33.1 release base. #17 remains the historical review record; this PR is the active v0.33.1 review.