[benjaminkott]

Description

Master becomes 17.0 and follows TYPO3 main: the package declares
^14.3 || 15.*.*@dev and drops 13.4, the way the previous major
crossing dropped 12.4. Everything the branch removes or changes comes
from what TYPO3 15 removed (Breaking-109783 and its siblings) and from
the deprecation log of the installation on both majors, which is empty
afterwards.

One commit per concern:

• Drop v13 and allow v15@dev — constraints, ext_emconf.php,
  CI matrix (15 runs on PHP 8.5 only), README, docs. b13/container
  leaves require-dev: it has no release for 15, so a composer update against main could not resolve. The ^14.3 CI cells add it
  and PHPStan checks the container registration there.
  composer typo3:dev resolves the dependencies against the
  development major without touching composer.json; the CI uses the
  same --with.
• Drop the TYPO3 v13 fallbacks — the < 14 branches in
  ext_localconf.php, the test skips, the routing note, the 13
  PHPStan baseline.
• Upgrade wizards on the EXT:core interfaces — the EXT:install
  ones are gone in 15.
• FlexForms through columnsOverrides — addPiFlexFormValue() is
  gone in 15.
• csp instead of useNonce on f:asset.css.
• FlexFormTools instead of FlexFormService — the alias is gone.
• [BUGFIX] Image dimensions from the processed files — 15 stops
  reporting media to the AssetCollector, <img width="" height="">
  was the result. A listener on AfterFileProcessingEvent collects
  them on every declared major; the ViewHelper keeps its API.
• ds instead of ds.default — the TCA migration 14 logs.
• PHPStan without b13/container — path-scoped ignore.
• [BUGFIX] Page layout column identifiers — 15 throws for a
  column without one, 14 derived a hash and logged; every page of
  the site answered 500. 108 columns, identifier = label suffix.
• Sass values for scssphp — raw variables, url() without an
  argument declaration and with a PHP return value are deprecated in
  scssphp 1.x and gone in 2.0.
• Package resources through the package manager — the page
  renderer hands PKG: identifiers to the compile hook, and
  getFileAbsFileName() deprecates resolving those.
• Core labels by their current domains — 17 labels the TCA
  borrowed from locallang_ttc.xlf are gone in 15; every palette
  heading in the forms was empty. core.form.palettes and
  frontend.db.tt_content carry the same strings on both majors.

Type of change

• Bugfix
• Task
• Feature
• Documentation

Related issues

Needs bk2k/extension-helper 2.3 (released) for Symfony Console 8.
b13/container has no release for TYPO3 15; the CI leaves it out of
that cell until it has.

How to validate

Both majors were run as Branchery worktrees with a copy of the
development database — TYPO3 dev-main ccdcf13 on PHP 8.5.9, and
14.3.7 with b13/container 3.2.3:

1. composer test:php:lint, composer cgl:ci, composer phpstan,
   composer test:php:unit (96), composer test:php:functional (74)
   — green in both.
2. All 45 pages of the site answer 200 with their images.
3. Backend: page module on 9 pages covering all 13 layouts, edit forms
   of 9 element types with their FlexForm fields and palette headings.
4. var/log/typo3_deprecations_*.log is empty after 2 and 3, on 14.3
   and on 15.
5. composer typo3:dev on a checkout resolves to dev-main.

Left as is: 10 PHP 8.5 deprecations of scssphp's own signatures and 18
PHPUnit 13 notices about mocks in the unit suite (pre-existing), and
the Extension Scanner was not run — the identifier sweep over
Breaking-109783 stands in.

AI assistance

• Agent and version: Claude Code, VS Code extension
• Model and effort/reasoning level: Claude Opus 5 (1M context), default
  effort
• Share written by the agent: the whole change and this description
• Reviewed and understood before pushing: the frontend and backend of
  the v15 worktree were checked by hand; code review pending

Checklist

• Targets master
• Commits are signed off (git commit -s)
• Commit subjects follow [BUGFIX|TASK|FEATURE] Subject
• composer cgl leaves the code unchanged
• composer phpstan passes
• composer test passes
• A bugfix comes with a test that fails without it — both
  bugfixes (GalleryTest, BackendLayoutsTest)
• Holds on every TYPO3 and PHP version declared in composer.json
  — 14.3 and 15 locally on PHP 8.5, the rest is the CI matrix
• Assets rebuilt — no SCSS, JavaScript or icons changed
• Documentation updated — installation requirements, contribution
  guide, routing note