make opening index size inspection O(1)

[zehiko]

Problem

LanceIndexStore::deep_size_of_children counted object_store + index_dir + metadata_cache. object_store and metadata_cache are Arcs to process-wide shared infrastructure, not data the index owns.

A cached BTreeIndex reaches its LanceIndexStore (and thus metadata_cache) via Arc, so moka's byte-weigher — which calls deep_size_of() on every index-cache insert — walked the entire shared metadata cache to weigh a single index. Two consequences:

1. Cost: each scalar-index open became O(metadata-cache size) (LanceCache::deep_size_of_children → approx_size_bytes() iterates every entry). On a large shared cache this dominates CPU.
2. Mis-weighting: every index entry was charged ≈ the whole metadata cache, inflating and equalizing index-cache entry weights, so size-based eviction evicted index objects almost immediately and re-opened them on nearly every query.

In production this showed up as open_scalar_index on essentially every query, each paying the O(metadata-cache) walk.

Fix

Count only data the store owns: index_dir + file_sizes. This mirrors BTreeIndex::deep_size_of_children, which already skips its index_cache field to avoid circular references.

It deliberately does not touch approx_size_bytes / LanceCache::deep_size_of_children (no weighted_size), so it carries no telemetry-accuracy tradeoff — the per-open path simply never reaches the cache.

Testing done

Microbenchmark timing the real LanceIndexStore::deep_size_of() vs metadata-cache fill N (release-7.0.0 vs this branch, same harness):

N (metadata entries)	release-7.0.0	this fix
1	5.6 µs	~1 ns
1,000	91 µs	~1 ns
10,000	767 µs	~1 ns
100,000	19.5 ms	~1 ns

O(N) → O(1), independent of cache fill.

• stack testing - WIP

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[zehiko]

This path is no longer relevant after Lance lance-format#6222, see details in https://linear.app/rerun/issue/RR-4918/maven-slow-query-investigation