fix(parsers/php): resolve $this->/self:: calls into used traits

[gadievron]

fix(parsers/php): resolve $this->/self:: calls into used traits

Base: pr/zig-php-generic-anon · Depends-on: #127 · Type: bug fix · Finding: F12 (HIGH)

Stacked on our own in-flight PR #127 (pr/zig-php-generic-anon), which already
lands the prerequisite PHP-parser fixes (untagged-fragment <?php prepend in
_extract_calls_from_code; relative_scope handling for self::/static::/
parent:: in _resolve_scoped_call). This PR adds ONLY the net-new
trait-composition delta — no overlap with #127's hunks.

What

• parsers/php/function_extractor.py — capture in-class use_declaration trait
  names onto a new traits field on the class record (_extract_trait_names).
• parsers/php/call_graph_builder.py — build a traits_by_class index and make
  _resolve_self_call fall back to the used traits' methods when no own method
  matches.

Why

$this->method() / self::method() calls into a method defined in a used trait
were never resolved: _resolve_self_call only indexed methods physically in the
class body, and trait use was never recorded as a class→trait composition. Trait
methods live under the trait's own key, so the edges were dropped. Measured: of
trait methods, symfony 77% orphaned (425/553), laravel 68%, composer 71%, guzzle
86% — a major contributor to PHP's high isolated-function ratio.

Tests

tests/test_php_trait_self_call.py (new): trait T{ g() }, class
C{ use T; f(){ $this->g(); } h(){ self::g(); } }; asserts edges C::f→T::g and
C::h→T::g (builder-dict + end-to-end extractor layers).

• RED (on the fix(zig,php parsers): attribute generic-container & anonymous-class methods correctly #127 base): edges missing ('a.php:T.g' not in []).
• GREEN: 2 passed; full PHP suite 46 passed (incl. test_php_schema_completeness
  — the new traits field doesn't break the schema).

Scope / siblings

Ruby has the same shape (include/extend/prepend mixins never recorded as a
class→module composition; _resolve_self_call has no mixin fallback) — confirmed
sibling, tracked separately, not fixed here.

Upstream coordination

No open PR adds trait-composition resolution. PR #122 (php/ruby) handles only
conditional-branch same-name collisions, not traits — non-overlapping. Must merge
after #127.

Author notes

• Fix lines: traits capture in function_extractor.py; traits_by_class +
  _resolve_self_call trait fallback in call_graph_builder.py.
• Input it now handles: $this->g() / self::g() where g is defined in a
  used trait.
• Likely pushback: "why stacked, not vs master?" — the two prerequisite fixes
  live in fix(zig,php parsers): attribute generic-container & anonymous-class methods correctly #127; rebasing onto master would duplicate/conflict with them.

[gadievron]

Finding F12 (HIGH). ⚠️ Stacked on #127 (base pr/zig-php-generic-anon). #127 already lands the prerequisite PHP fixes (<?php tagless-parse, relative_scope); this PR adds ONLY the net-new class→trait composition index + _resolve_self_call trait fallback (symfony had 77% of trait methods orphaned). No overlap with #127's hunks. Merge after #127.