feat(streaming): SST deduplication and style registry in streaming writes

[arcaputo3]

Summary

Streaming writes currently emit inline strings with no SST deduplication and default styles only. Add two-phase streaming write support for shared strings and cell formatting.

Current State

• Streaming reads: production-ready, O(1) worksheet memory via SAX
• Streaming writes: functional but limited — inline strings only, no rich formatting
• StreamingXmlWriter uses BoundsAccumulator for dimensions but no SST/style phases

Proposed Approach (from smart-streaming design doc)

Phase 1 — SST dedup:

• First pass: collect unique strings, build SST index
• Second pass: emit worksheet XML with SST references instead of inline strings
• Result: smaller files, Excel-standard SST usage

Phase 2 — Style registry:

• StyledRowData type carrying cell values + style IDs
• Build StyleRegistry during streaming, emit styles.xml
• Enable formatted streaming output (number formats, fonts, fills, borders)

Phase 3 — Merged cells:

• Emit <mergeCells> from streaming writers (currently in-memory only)

Impact

Enables large file creation (100k+ rows) with proper formatting — currently requires full in-memory path.

Consolidates Linear TJC-319, TJC-320, TJC-486.