feat: add multimodal search and granular performance diagnostics

crates/searchi-semantic/README.md

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+# Google worker image decoding
+
+The production adapter verifies model/projector SHA256 and performs genuine
+Google MTMD image/audio inference. Images are decoded with established Rust
+libraries, not filename captions. `WorkerInfo.media_supported` comes from the
+native handshake; absent fields on older text workers mean false. Media APIs
+return an explicit runtime repair error for such workers while text remains
+compatible.
+
+JPEG sources up to 64 Mi pixels (including 48 MP camera photos), 16,384 pixels
+per side, and 64 MiB compressed size are supported. For JPEG above 16 Mi pixels,
+`image-rs/jpeg-decoder` 0.3.2 applies scaled IDCT before final model resizing.
+Its default features are disabled, so this path adds no Rayon worker pool.
+The source dimensions are checked before pixels are decoded. The secondary
+header must match the first to detect file changes. RGB24 and grayscale camera
+JPEGs are supported on this scaled path; unusual large CMYK/16-bit JPEGs fail
+explicitly rather than silently taking an unbounded full-resolution fallback.
+
+The published decoder offers 1/8, 1/4, 1/2, and full-scale decoding. It is in
+maintenance mode; the current image 0.25 zune JPEG adapter exposes no scaled
+IDCT interface. See the [decoder API](https://docs.rs/jpeg-decoder/0.3.2/jpeg_decoder/struct.Decoder.html)
+and [upstream maintenance note](https://github.com/image-rs/jpeg-decoder).
+Small JPEGs retain the existing image/zune path and pinned parity behavior.
+
+PNG, WebP, and BMP remain capped at 16 Mi pixels with explicit diagnostics.
+Both paths enforce a 256 MiB advertised decoding-buffer ceiling and apply
+EXIF orientation. That ceiling is a buffer/output bound, not an absolute
+process RSS guarantee: compressed data, codec working state, and resizing
+have additional allocations. Extreme/progressive files need broader resource
+profiling. The media fingerprint includes `largejpeg-scaled-v2`; generations
+must additionally include visual token budget. Text fingerprints are unchanged.
+
+An executed Windows CPU release test encodes and decodes an actual 8000x6000
+JPEG with EXIF rotation, verifies orientation and region colors, and resizes it
+to a 280-token grid. Decode plus resize took 176–220 ms and produced 672x912
+RGB (1,838,592 bytes). A sampled whole-test peak was 146.86 MiB, including
+144 MB of source fixture construction; it is not a decoder-only measurement
+or a semantic quality benchmark. An 81 MP JPEG header is rejected before
+pixel decode in the default tests.
+
+```powershell
+cargo test --release -p searchi-semantic large_phone_jpeg -- --ignored --nocapture
+cargo test -p searchi-semantic --lib
+```