feat: add multimodal search and granular performance diagnostics

crates/searchi-service/README.md

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+# Shared hybrid retrieval
+
+`HybridSearch` combines genuine lexical and Google multimodal retrieval. The
+CLI and desktop use the same service; it has no desktop-shell dependency.
+
+`search`, `search_semantic` and `search_semantic_with_origin` preserve grouped
+parent files. ANN candidates expand until enough unique eligible parents exist
+or the complete bounded eligible chunk set is requested. Filename/type/folder,
+size/date/duration and explicit source filters apply before ANN top-k. Media
+candidate filesystem metadata is checked once per parent per query. Ninety-six
+near-identical video vectors cannot hide a second eligible parent (regression
+tested). HNSW remains approximate retrieval, not an exact recall guarantee.
+
+Explicit spoken/audio wording prefers actual audio-source vectors, preserving
+exact lexical filename priority. Conservative English/German token cues detect
+requests such as “someone mentions”, “spoken”, “audio”, “jemand erwähnt” and
+“Gespräch”. Visual/document cues veto automatic routing. This finite parser is
+not language understanding; paraphrases, negation and mixed intents can remain
+ambiguous. If the preferred lane has no eligible results, ordinary retrieval
+is used. `SemanticOrigin::All` bypasses the hint; `Audio`, `Image`, `Video` and
+`Text` select an explicit channel without fallback or reembedding.
+
+Persisted custom sampling limits can be reopened without generating embeddings
+again only when the complete model and preprocessing fingerprint matches.
+The custom suffix must contain the eight valid numeric sampling limits. Model
+or preprocessing changes never silently reuse incompatible vectors.
+
+`search_measured(query,limit,options,debug)` returns `SearchResponse` with results
+and monotonic total milliseconds/result count. Basic mode stores no trace.
+`search_traced(...,&mut TraceCollector,parent)` shares the native request clock
+so queue/lock time and actual engine stages remain comparable. Debug mode adds
+bounded nested spans for lexical SQL, query embedding, worker diagnostics,
+filtered eligibility and its cache, ANN read/checksum/deserialization/search
+passes, parent materialization and validation, and rank fusion. Static stage
+names and numerical counters exclude query/source text and paths. Worker-local
+microsecond counters have a separate clock and are not aligned request spans.
+
+Measured/source APIs retain existing result behavior except the explicit
+spoken/audio preference. Debug instrumentation does not change vector policy.
+Audio timestamps represent segment starts, not exact word locations. Smaller
+windows alone have not demonstrated accurate spoken navigation. No-answer
+calibration, music discrimination, exact spoken positions, process isolation,
+unchanged-vector reuse and production platform checks remain release gates.