feat: add register_statement_class API for extending PPI grammar

[toddr-bot]

Summary

Adds a public API (PPI::Lexer->register_statement_class()) that allows external code to register custom keywords (like async, method, around) that should be mapped to specific statement classes during lexing. This addresses the long-standing request to make PPI's grammar extensible without monkey-patching internals.

Fixes #270

Changes

• PPI::Lexer: Add register_statement_class($keyword, $class) and unregister_statement_class($keyword) class methods that modify the internal %STATEMENT_CLASSES hash
• PPI::Statement::Sub::name(): Handle prefix keywords before sub — when the expected name position holds sub (e.g., async sub hello {}), skip past it to find the actual name
• PPI::Token::Whitespace: Relax prototype detection to recognize prototypes after any word preceding sub name (, not just my/our/state
• Tests: 8 new tests covering the API, prefix+sub patterns, sub-like keywords without sub, anonymous prefix subs, and cleanup verification

Example usage

use PPI;
use PPI::Lexer;

# Register 'async' as a sub-like keyword
PPI::Lexer->register_statement_class('async', 'PPI::Statement::Sub');

my $doc = PPI::Document->new(\"async sub hello { ... }");
my $sub = $doc->child(0);
# $sub isa PPI::Statement::Sub
# $sub->name returns 'hello'
# $sub->block returns the PPI::Structure::Block
# $sub->prototype works correctly

# Also works for sub-like keywords without 'sub':
PPI::Lexer->register_statement_class('method', 'PPI::Statement::Sub');
my $doc2 = PPI::Document->new(\"method hello { ... }");
# $doc2->child(0) isa PPI::Statement::Sub, name() returns 'hello'

# Clean up when done
PPI::Lexer->unregister_statement_class('async');

Test plan

• All 8 new tests pass (API, async sub with block/forward/prototype/attributes, method keyword, anonymous async sub, cleanup)
• Full test suite: 53089 tests across 69 files, all passing, zero regressions
• Verified round-trip safety preserved

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Generated by Kōan /fix

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Quality Report

Changes: 4 files changed, 155 insertions(+), 8 deletions(-)

Code scan: clean

Tests: skipped

Branch hygiene: clean

Generated by Kōan post-mission quality pipeline