Fix OOXML schema validity, correctness, perf, and security issues

[SimonCropp]

Correctness:

• Tables: derive column count from the rowspan grid so cells in later, wider rows are no longer silently dropped
• Tables: thread FootnoteIndex/BookmarkId through per-cell contexts and seed FootnoteIndex from the existing part, so footnote and bookmark IDs no longer collide across cells or across Build calls
• Emit w:rPr and w:tcPr children in OOXML schema order; previously the output was schema-invalid and only tolerated by Word
• Cell margins emit w:left/w:right instead of the invalid w:start/w:end
• ColorParser: accept percentage and decimal rgb()/rgba() components
• vertical-align is matched case-insensitively
• word-spacing no longer doubles letter-spacing
• @page: match the closing brace and skip nested margin-box rules so sibling declarations (margin/size) are not lost

Performance:

• Parse each element's inline style once instead of twice
• Allocate list numbering IDs from the threaded NextNumId counter rather than rescanning the numbering part per list (O(m^2) -> O(m))
• Compute ordered-list item indices in a single pass (O(n^2) -> O(n))

Security:

• Image fetch: disable auto-redirect and re-check each redirect hop against the image policy (closes a SafeDomains/SSRF bypass), cap the response size, set a timeout, and parse width/height with InvariantCulture

Tests:

• Add rowspan, footnote-id, bookmark-id, and OpenXmlValidator round-trip tests, plus rgb() percentage cases
• Regenerate affected verified baselines (structural reordering only; rendered text/image output unchanged)