Summary

This PR improves formula cached-value writeback correctness and adds explicit APIs for typed cached results.

It is intentionally split into two commits:

1. fix(cell): serialize formula cache with correct cell types
2. feat(cell): add typed formula cached-result setters

Motivation

This work is grounded in a concrete downstream integration need from Formualizer.

Formualizer recently added Rust-native workbook recalculation/writeback (recalculate_file) using umya-spreadsheet as the writer path. During validation, formula cached values written back to XLSX were observed as string-typed payloads in XML (t="str") even when the computed result was numeric, boolean, or error.

That behavior creates type drift in sheet*.xml and can degrade interoperability with Excel/OpenXML consumers that rely on correct cached value typing.

Additionally, Formualizer needed an explicit, type-safe way to set formula cached results (number/bool/error/string/blank) while preserving formula metadata.

While this was identified through Formualizer, the fix is broadly useful: it improves formula-cache correctness for all umya-spreadsheet users.

What changed

1) Typed formula cached-value serialization

• Formula cells now emit type according to cached raw value kind:
  • numeric -> numeric path (no forced t="str")
  • bool -> t="b" + 1/0
  • error -> t="e" + actual error token
  • string -> t="str"
  • blank -> empty value
• Error serialization now writes the actual cached error value (e.g. #N/A) instead of hardcoding #VALUE!.

2) New typed API helpers (backwards-compatible)

Added on both CellValue and Cell:

• set_formula_result_number(...)
• set_formula_result_bool(...)
• set_formula_result_error(...)
• set_formula_result_string(...)
• set_formula_result_blank()

These methods update cached value (<v>) without removing formula metadata (<f>).

Existing set_formula_result_default(...) remains unchanged.

Tests

Added regression coverage for:

• formula cached-value type mapping
• typed XML output for number/bool/error/string/blank formula caches
• roundtrip read behavior
• shared-formula metadata stability after write
• typed helper behavior preserving formula text

Commands run

• cargo test (full suite) ✅
• targeted tests for new scenarios ✅

Note: clippy -D warnings currently has pre-existing baseline failures on upstream master unrelated to this PR. This PR does not introduce new clippy debt in touched logic.

Backwards compatibility

• No breaking API changes.
• Existing methods and call patterns continue to work.
• Output changes are correctness-oriented (typed cache serialization) while preserving formula objects/metadata.

Why this helps

• Fixes a real production integration path in Formualizer (single-call workbook recalc + cache writeback).
• Better Excel/OpenXML interoperability via correctly typed formula cached values.
• Cleaner writeback semantics for any engine that computes and persists formula caches.
• More explicit and safer API for setting cached results programmatically.