This implements a high-performance bitmap index for tool filtering:- ToolBitmap: 256-bit bitmap (4 uint64s) with O(1) set operations- ToolIndex: Pre-computed index for toolset, read-only, and feature flag filtering- Query: Returns guaranteed tools + those needing dynamic checks- Materialize: Only runs dynamic Enabled() checks on survivorsPerformance benchmarks:- Query (2 toolsets): 78 ns/op, 0 allocs- Query (all toolsets): 44 ns/op, 0 allocs- Bitmap OR/AND: <0.3 ns/op- Full query + materialize (130 tools): 5.4 µs/op, 4 allocsKey insight: Static filters (toolset, read-only, feature flags) can bepre-computed as bitmaps. Dynamic Enabled() checks are only run on toolsthat survive static filtering, avoiding wasteful checks on tools thatwould be filtered out anyway.

Adds two new methods to ToolIndex:- UniqueFeatureFlags(): Returns all unique feature flags used by tools- QueryWithFeatureChecker(): Queries with automatic flag checkingKey optimization: Instead of checking the same feature flag N timesfor N tools that use it, each unique flag is checked exactly once.For example, if 50 tools use 3 unique flags, this reduces feature flagservice calls from O(50) to O(3).Test demonstrates: 50 tools using 3 flags results in exactly 3 checks.

Change Materialize() to return []*ServerTool instead of []ServerTool.This avoids copying the entire ServerTool struct (which includes theJSON schema) for each tool in the result.Benchmark improvement:- Materialize: 6199ns → 1114ns (5.6x faster)- Memory: 12584B → 472B (27x less)- Allocations: 4 → 3The returned pointers reference the cached tools in the index.Callers must not modify them.

Provides a lightweight mechanism for tools that have different definitionsbased on runtime conditions (e.g., feature flags, capabilities).Designed for the small number of tools (~2) that need variant handling:- ToolOverride: condition + replacement definition- ToolOverrides: map[string]ToolOverride with Apply() and ApplyToTools()- Lazy allocation: no allocs when no overrides matchThis is intentionally simple - only 60 lines - because most tools don'tneed variants and the full VariantIndex would be overkill.