Avoid quadratic index updates in uv add

[zaniebot]

Adding many --index values with uv add repeatedly scans, reparses, canonicalizes, clones, removes, and repositions the full [[tool.uv.index]] array for each incoming index. The repeated work makes large index updates quadratic before resolution begins.

Batch index updates and preserve existing table formatting, priority, credential redaction, and path matching. Keep the flat/simple synchronization from uv#19818. If uv-dev#57 lands first, preserve its source-reference rename when batching updates.

Performance

A Criterion size sweep on a dedicated 16-vCPU Namespace runner with CPU affinity and an optimized Rust 1.97 build shows the individual path growing quadratically and the batched path linearly:

Indexes	Individual updates	Batched update	Improvement
32	436.11 µs	33.421 µs	13.0×
128	6.8681 ms	134.41 µs	51.1×
512	111.01 ms	554.37 µs	200×
2,048	1.7887 s	2.3203 ms	771×