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 the project's supported PHP versions
(7.4+), 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).
• Content that the basic format cannot represent — non-text shapes (images,
  lines, tables, charts) and in-run line breaks — is silently skipped on
  save; this is characterized by tests (see below).
• Styling (font/bold/size/color), 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.
• Reader coverage includes canRead() rejection of non-Keynote files, non-Zip
  files and directories, and load() error cases (missing file, wrong format).
• Round-trip coverage includes: multi-slide content with geometry, paragraphs,
  runs and speaker notes; an empty presentation; Unicode and XML metacharacters
  (< > & " ', non-ASCII, emoji) surviving escaping unchanged; multiple text
  placeholders per slide preserved in order; and non-representable content
  (a line shape and an in-run line break) being skipped without data corruption.
• 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.

[coveralls]

coverage: 92.141% (-0.006%) from 92.147% — yasumorishima:feature/keynote-reader-writer into PHPOffice:master

[yasumorishima]

Rebased onto master now that #897 is merged.

The two PHP Static Analysis (8.4) / (8.5) failures on this PR came from PHPStan 1.x being unable to resolve PHPUnit 13 symbols on PHP 8.4/8.5, which #897 fixed by moving to PHPStan ^2.2. After rebasing, PHPStan 2.2.8 at level 6 (no baseline) reported three issues in this PR's own files, all fixed here:

• Writer/Keynote.php: dropped the '' == $pFilename half of the tempnam() guard — tempnam() returns non-falsy-string|false, so the comparison was always false.
• Tests/KeynoteRoundtripTest.php: removed two instanceof RichText guards that follow an assertInstanceOf(RichText::class, ...) — they were always true, and calling the getters directly makes the assertions unconditional.

The presentation diff is otherwise unchanged from the previous head.