Add Keynote reader and writer

[yasumorishima]

What it does

Adds a Reader and Writer for the Apple Keynote (.key) presentation format.

The implementation targets the legacy iWork '09 structure, where a Keynote
package is a Zip archive containing an index.apxl XML document. Both classes
follow the existing reader/writer conventions (ReaderInterface / AbstractWriter)
and are registered in IOFactory for automatic resolution.

Supported in this first milestone:

• Slides
• Text placeholders — paragraphs and runs
• Shape geometry (position and size)
• Speaker notes

Why

Resolves the long-standing request for Keynote support (#48). A previous attempt
(#860) targeted the modern IWA binary format and stalled; this PR deliberately
scopes to the XML-based iWork '09 structure so it can be implemented in pure PHP
(no new dependencies), stays compatible with PHP 7.1+, and is fully verifiable in
CI.

Scope / limitations (intentional)

• The modern IWA binary format (Keynote '13 and later: Snappy-compressed
  Protobuf) is out of scope and is safely rejected by canRead() (no false
  positives).
• Because of the format generation gap, output is not guaranteed to open
  directly in a current Keynote.app. Correctness is instead guaranteed by a
  round-trip test (write → read → assert structure).
• Styling (font/bold/size/color), images, tables, charts, themes and document
  properties are left as follow-up work.

How it was tested

• New unit tests for the reader and writer, plus a dedicated round-trip test
  (including empty-presentation and canRead() rejection cases for non-Keynote
  files, non-Zip files and directories).
• phpunit, phpstan (level 6), php-cs-fixer and phpmd all pass; the full
  existing suite remains green (no regressions).

Notes

• No new runtime dependencies; uses only ext-xml and ext-zip (already required).
• Documentation updated in docs/usage/readers.md and docs/usage/writers.md.

Closes #48

/claim #48

[yasumorishima]

A quick note on the currently red CI jobs — they are all unrelated to this change:

• PHPUnit / PHPStan / samples on PHP 7.2 & 7.3 fail at composer install: dependencies can no longer be resolved because phpoffice/phpspreadsheet now requires PHP >= 7.4. These jobs fail before running anything, and this affects the repository's CI in general on those versions (not specific to this PR).
• PHPStan on PHP 8.4: with no committed composer.lock, the latest phpunit/phpunit (13.x) is installed, which PHPStan then reports as extends unknown class PHPUnit\Framework\TestCase across all existing test classes (e.g. AbstractShapeTest). This is an environment/tooling issue, not something introduced here.
• PHPUnit 8.1: a single pre-existing failure in Writer\ODPresentation\ObjectsChartTest::testTypeAxisUnit, unrelated to Keynote.

The new Keynote reader/writer is green on every job where the environment resolves correctly: PHPStan 7.1 / 7.4 / 8.0–8.3, PHPUnit 7.1 / 7.4 / 8.0 / 8.2 / 8.3 / 8.4, all samples jobs, PHP-CS-Fixer and PHPMD.

Happy to adjust anything on request.