This commit removes the runtime rewrite binary and replaces it with build-time
placeholder replacement, fixing multiple issues with multi-buildpack deployments
and git URL buildpack usage. It also fixes a critical bug where php.ini.d
configs were processed with the wrong HOME context.

## Rewrite Binary Removal

The rewrite binary was originally copied from the v4.x Python buildpack and used
to replace template variables in configuration files at runtime. This approach
had several issues:
- Failed when buildpack was deployed via git URL
- Compilation errors in multi-buildpack scenarios
- Security concerns with runtime config rewriting
- Performance overhead at container startup

This commit completes the migration to build-time placeholder replacement that
provides better security, performance, and multi-buildpack compatibility.

### Changes:
- Remove bin/rewrite shell wrapper script
- Remove src/php/rewrite/cli/main.go (entire rewrite implementation)
- Remove rewrite binary compilation from bin/finalize
- Remove bin/rewrite from manifest.yml include_files
- Remove /bin/rewrite-compiled from .gitignore
- Update ARCHITECTURE.md to remove rewrite binary documentation

## Build-Time Placeholder Replacement

Add ProcessConfigs() method to finalize phase that replaces @{VAR} placeholders
with actual values during staging:

- @{HOME} - App or dependency directory path
- @{DEPS_DIR} - Dependencies directory (/home/vcap/deps)
- @{WEBDIR} - Web document root (default: htdocs)
- @{LIBDIR} - Library directory (default: lib)
- @{PHP_FPM_LISTEN} - PHP-FPM socket/TCP address
- @{TMPDIR} - Converted to ${TMPDIR} for runtime expansion
- @{PHP_EXTENSIONS} - Extension directives
- @{ZEND_EXTENSIONS} - Zend extension directives

## Placeholder Syntax Unification

Unify all template variables to use @{VAR} syntax consistently across all
config files (httpd, nginx, php-fpm, php.ini).

## Multi-Buildpack Fixes

- Fix PHP-FPM PID file path to use deps directory for multi-buildpack scenarios
- Fix nginx configuration for Unix socket and runtime variable expansion
- Update supply buildpack integration tests

## php.ini.d Context Bug Fix

Fix critical bug where php.ini.d directory was processed with deps context
(@{HOME} = /home/vcap/deps/{idx}) instead of app context (@{HOME} =
/home/vcap/app).

The php.ini.d directory contains user-provided PHP configurations that typically
reference application paths (include_path, open_basedir, etc.), similar to fpm.d
configs. Processing with deps context caused the buildpack-created
include-path.ini and user configs to reference incorrect paths.

### Changes:
- Process php.ini.d separately (like fpm.d) with app-context replacements
- Update supply.go comments to clarify php.ini.d context behavior
- Add fixture test for @{HOME} placeholder in php.ini.d configs
- Enhance modules integration test to verify placeholder replacement

Both fpm.d and php.ini.d now use app HOME context while other PHP configs
(php.ini, php-fpm.conf) use deps HOME context.

Fixes issues with:
- Buildpack-created include-path.ini referencing wrong directory
- User-provided php.ini.d configs using @{HOME} placeholders
- Include paths not resolving to application lib directory

Create a symlink from WEBDIR/vendor to the actual vendor directory during
Composer compilation. This allows apps to use relative require paths like
`require 'vendor/autoload.php'` from their web root (e.g., htdocs).

The symlink is only created when:
- WEBDIR exists (e.g., htdocs directory)
- Vendor directory is not already inside WEBDIR
- No existing vendor directory or symlink exists in WEBDIR
- Actual vendor directory exists after Composer installation

Example: If composer.json specifies vendor-dir as "lib/vendor" and WEBDIR is
"htdocs", this creates htdocs/vendor -> ../lib/vendor

This maintains backward compatibility for apps that reference vendor/autoload.php
from their web-accessible directory while keeping the actual vendor directory
outside the web root for security.

Also fixes placeholder syntax: #{LIBDIR} -> @{LIBDIR} to match the unified
@{VAR} syntax used throughout the buildpack.

This commit improves the integration test infrastructure with better cleanup,
more robust regex matching, and proper handling of unsupported features.

Changes:
- Fix integration test regex patterns for more reliable matching
- Skip custom extensions test (feature not yet supported in v5.x)
- Cleanup buildpack files after integration tests to prevent disk space issues

Test Infrastructure Improvements:
- Add buildpack cleanup in init_test.go to remove uploaded buildpacks after tests
- Refactor default_test.go regex patterns for better readability
- Add explicit skip for custom extensions with clear explanation

These changes improve test reliability and reduce CI/CD resource usage by
properly cleaning up test artifacts.

Add 80K of comprehensive documentation covering architecture, features, migration,
and service binding patterns for both end users and developers/maintainers.

Documentation Structure:
- docs/USER_GUIDE.md (15K, 865 lines) - Complete end-user guide
- docs/FEATURES.md (11K, 696 lines) - Developer reference with test coverage
- docs/BUILDPACK_COMPARISON.md (12K) - Cross-buildpack architectural analysis
- docs/VCAP_SERVICES_USAGE.md (12K) - Service binding patterns and best practices
- docs/REWRITE_MIGRATION.md (27K) - v4.x to v5.x migration guide
- docs/README.md (7K) - Navigation hub with best practices

USER_GUIDE.md (For End Users):
- Getting started (deploy in 2 commands)
- Web server configuration (Apache, Nginx, FPM-only)
- PHP configuration (versions, ini files, FPM pools)
- PHP extensions installation
- Composer and dependency management
- APM integration (NewRelic, AppDynamics, Dynatrace)
- Session storage (Redis, Memcached)
- Framework guides (Laravel, CakePHP, Symfony, Laminas)
- Advanced features (multi-buildpack, preprocess commands)
- Troubleshooting section

FEATURES.md (For Developers/Maintainers):
- 30+ features with explicit integration test references
- Test locations (file:line numbers)
- Fixture paths for each feature
- Implementation details and code snippets
- Test coverage analysis matrix
- Identification of features needing explicit tests
- Cross-references to integration tests

BUILDPACK_COMPARISON.md:
- Proves PHP v5.x follows same patterns as Go, Java, Ruby, Python buildpacks
- Documents VCAP_SERVICES availability during staging
- Explains why v4.x runtime rewrite was PHP-unique, not a CF standard
- Shows alignment with Cloud Foundry buildpack ecosystem

VCAP_SERVICES_USAGE.md:
- Clarifies VCAP_SERVICES IS available during staging (for extensions and Go code)
- Documents that @{VCAP_SERVICES} config file placeholders are not supported
- Provides service binding patterns and examples
- Migration strategies from v4.x
- Best practices and anti-patterns

REWRITE_MIGRATION.md:
- Comprehensive v4.x to v5.x migration guide
- Breaking changes in rewrite system
- Placeholder syntax changes
- User-provided config handling
- Corrects misconceptions about VCAP_SERVICES availability

Key Documentation Insights:
- VCAP_SERVICES IS available during staging in Go code (not just runtime)
- PHP v5.x aligns with all other CF buildpacks (Go, Java, Ruby, Python)
- v4.x runtime rewrite was PHP-unique, removed for CF standards alignment
- 95%+ test coverage verification for documented features
- Separate docs for end users vs developers/maintainers

The documentation demonstrates comprehensive feature coverage and provides
clear guidance for both buildpack users and maintainers.

- Add {PHP_XX_LATEST} placeholder resolution in options.json
  - Resolves {PHP_83_LATEST}, {PHP_82_LATEST}, {PHP_81_LATEST} to actual versions
  - Provides clear error messages for invalid placeholders

- Add #{VAR} placeholder syntax for backward compatibility with v4.x
  - Support both @{VAR} and #{VAR} syntax in all config files
  - Restore compatibility for users migrating from v4.x buildpack

- Add comprehensive integration tests
  - 10 test cases covering placeholder resolution
  - Test fixtures for all scenarios
  - Unit tests for both features

Fixes #1208

…and security warnings

…v4.x parity