feat: write rich inline strings (per-run formatting) via Worksheet.inlineString

[minkyu725]

Add support for writing rich inline strings — a single cell with multiple
font styles (bold/italic/underline, font size/name/color per run).

Use case

Template headers and cells where one fragment must be highlighted relative
to the rest currently can't be expressed: Worksheet.value(r, c, String) /
Worksheet.inlineString(r, c, String) apply one font to the whole cell.
This adds a RichText value type and a Worksheet.inlineString(int, int, RichText) overload.

API

RichText rt = RichText.builder()
    .run("Pickable (A)").end()
    .run("\n*Direct contract only").bold().fontColor("FF70AD47").end()
    .build();
worksheet.inlineString(row, col, rt);

RichText, RichText.Builder, and RichText.RunBuilder are public.
RichText.Run constructor is package-private — runs are produced through
the builder. Unset font properties on a run are omitted from <rPr>,
letting Excel inherit from the cell style.

Wire format

Per OOXML (ECMA-376 §18.4):

<c r="A1" t="inlineStr">
  <is>
    <r><t xml:space="preserve">Pickable (A)</t></r>
    <r><rPr><b/><color rgb="FF70AD47"/></rPr><t xml:space="preserve">
*Direct contract only</t></r>
  </is>
</c>

xml:space="preserve" is always emitted so leading/trailing whitespace and
newlines round-trip correctly.

Tests

Three POI round-trip cases added to PoiCompatibilityTest:

• per-run formatting (bold + custom color) preserved
• leading/trailing whitespace preserved
• runs without any formatting properties produce a bare <r><t>...</t></r>

All 108 existing tests in fastexcel-writer continue to pass.

Scope

This PR touches fastexcel-writer only. Issue #382 requests the
reader module expose rich text formatting when parsing an existing
xlsx — that path is unchanged here. Reader-side work would be a natural
follow-up but is out of scope (theme/indexed color resolution requires
loading theme1.xml, which the reader currently doesn't do — best
designed in a dedicated PR).

[coveralls]

coverage: 88.885% (-0.01%) from 88.896% — minkyu725:feat/rich-inline-string into dhatim:master