Optimize Sheet#rows_generator method to use less memory and parse faster

[connorshea]

After noticing that this method uses a lot of memory/takes a lot of time in the Rails app at work, I used Claude to optimize this and then verify that the output was identical from the released gem vs this change on a number of different files.

This should hopefully help resolve some long-standing issues with the performance/memory usage of the gem.

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Benchmarked on a generated 20,000-row x 40-col xlsx (mixed shared-string, numeric, and empty cells). Median full-parse time dropped ~1.72x (1.85s -> ~1.08s) and allocations dropped ~3.75x (28.6M -> 7.63M), with byte-identical output verified on both default- and prefixed-namespace files across rows, simple_rows, and rows_with_meta_data (including the self-closing-row + meta-data path).

Changes, in order of impact:

• Resolve the namespace prefix once via a namespace_resolved flag instead of re-checking node.namespaces on every node. Worksheets use a default namespace, so the old prefix.empty? guard never latched and allocated a namespaces hash for every node in the stream (660,002 calls -> 2). This is the bulk of the wall-clock win.
• Hoist node.name/node.node_type into locals (each was read up to 4x per node in the if/elsif chain) and reorder branches so the hottest nodes (<v>/<c>) are tested first.
• Fetch cell attributes with three individual node.attribute('t'/'s'/'r') C calls instead of node.attributes. Reader#attributes is attribute_hash.merge(namespaces), so it built and merged a namespaces hash on every call. With hundreds of thousands of cells the per-cell Hash allocation dominates, so three cheap lookups are both faster and leaner than building and indexing a hash.
• Use node.attribute_hash for the row-opener node (only ~one per row): this avoids the namespaces merge that node.attributes performs while preserving the exact meta-data row hash. Safe because xlsx row elements never declare their own namespaces (the namespace hash is empty for them), so the result is identical -- confirmed byte-for-byte including the self-closing-row + meta-data path.
• In fill_in_empty_cells, drop the redundant .to_a on the column range and use delete_suffix instead of gsub to strip the row number.

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Metric	Baseline	Optimized	Change
Median full-parse time	1.853s	~1.08s	~1.72× faster
Min full-parse time	1.827s	~1.04s	~1.76× faster
Allocations per parse	28.6M	7.63M	~3.75× fewer
Reader#namespaces calls	660,002	2	~330,000× fewer
Reader#attributes calls	660,000	0	eliminated

Stage	Median	Allocations
Baseline	1.853s	28.6M
+ namespace flag, hoist, reorder	~1.11s	~9.8M
+ attribute_hash (rows)	~1.10s	9.23M
+ node.attribute() per cell attr	~1.08s	7.63M
Total	~1.08s	7.63M