refactor(spec): cleanup pass over component specs

Address 8 issues found in spec audit (post PR #2):

1. §refs label: distinguish design vs report sections in p3-1 / p3-2 / p4-2 /
   p9-1 / p9-5 contract_sections (e.g., "report §11.2 Ollama" not "§11.2").
2. mock feature gate: gate MockEmbedder (p3-1) and MockLanguageModel (p4-1)
   behind `mock` cargo feature, default OFF; add CI symbol-scan as DoD item.
3. Warning type unification: p1-2 frontmatter now emits
   `kb_parse_types::Warning` (matches p1-3 / p1-4); drops crate-internal type.
4. p4-3 streaming thread: explicitly single-threaded inside RagPipeline::ask;
   collection + sink.send share the calling thread, no race. UI concurrency
   is callers responsibility (TUI worker thread pattern in p9-3).
5. p6-2 tesseract version: noted that `tesseract` 0.13 has no stable Rust
   `version()` accessor; use TessVersion FFI or shell-out + cache approach.
6. p9-* App struct extensions: introduce `kb_tui::{Library,Search,Ask,Inspect}State`
   slots in p9-1 forward-decl form; p9-2/3/4 fill bodies in their own crate
   without editing `App`. Parallel-safety contract added.
7. p3-3 cosine score: shift `(sim+1)/2` instead of clamp; preserve ranking
   signal between unrelated and opposite vectors. Clamp reserved for NaN.
8. fixtures/ root: p0-1 DoD now creates all fixture subdirs with .gitkeep so
   downstream tasks have a stable target path.

tasks/p4/p4-1-llm-trait.md

@@ -27,6 +27,7 @@ Provide the `kb-llm` crate that re-exports the `LanguageModel` trait and helper
 - `serde`
 - `thiserror`
 - `tracing`
+- `[features] mock = []` — opt-in feature flag exposing `MockLanguageModel`. Default OFF. Release builds compile mock out entirely.
 
 ## Forbidden dependencies
 
@@ -51,7 +52,8 @@ Provide the `kb-llm` crate that re-exports the `LanguageModel` trait and helper
 ```rust
 pub use kb_core::{LanguageModel, GenerateRequest, TokenChunk, FinishReason, TokenUsage, ModelRef};
 
-/// Test-only deterministic mock.
+/// Test-only deterministic mock. Compiled only when `mock` feature is on.
+#[cfg(feature = "mock")]
 pub struct MockLanguageModel {
     pub model_id: String,
     pub provider: String,
@@ -61,6 +63,7 @@ pub struct MockLanguageModel {
     pub canned_usage:  kb_core::TokenUsage,
 }
 
+#[cfg(feature = "mock")]
 impl kb_core::LanguageModel for MockLanguageModel { /* per §7.2 */ }
 ```