Reduce compilation time for feature-heavy builds

[Tristan-Stoltz-ERC]

Summary

Full-feature builds (--all-features) take significant time due to 98 feature flags and 55 workspace members. This issue tracks investigation and optimization of compile times.

Current state

• 98 feature flags (default = empty set)
• 55 workspace members (main crate + symthaea-core + 53 sub-crates)
• CI runs 18+ feature combination jobs in the matrix
• Key heavy features: lancedb-backend, neural-bridge, ssm_language, full_consciousness

Investigation areas

1. Feature unification — identify features that are almost always used together and merge them
2. Conditional compilation boundaries — ensure #[cfg(feature = "X")] is at module level, not sprinkled through functions
3. Parallel compilation bottlenecks — identify crates that serialize the build graph
4. Dependency deduplication — check for multiple versions of the same crate pulled by different features
5. Build script optimization — audit build.rs files across sub-crates
6. Type complexity — deeply nested generics slow rustc; identify hot spots with -Z time-passes

How to measure

# Time a full build from clean
cargo clean && time cargo build --all-features

# Identify slow crates
cargo build --all-features --timings

# Check dependency tree
cargo tree --duplicates

Constraints

• Do NOT use custom CARGO_TARGET_DIR — sccache handles caching
• One session per workspace to avoid cargo lock contention