feat: add four new native diagnostics

[calebdw]

Summary

This PR adds four new native diagnostics requested in issues #199, #278, #198, and #200. Each is a separate commit.

Enum declaration diagnostics (Closes #199)

Flags invalid enum declarations that would cause PHP fatal errors:

• Backed enum cases missing a value — a backed enum (enum Foo: int) where a case lacks = value.
• Unit enum cases with a value — a unit enum (enum Foo) where a case has = value.
• Invalid backing type — backing types other than int or string (e.g. enum Foo: bool).
• Duplicate backed values — two cases sharing the same backing value.

When an invalid backing type is detected, per-case value checks are skipped since they would be noise on top of the root cause.

Unimplemented trait abstract methods (Closes #278)

Concrete classes that use a trait with abstract methods without implementing them are now flagged as errors, matching PHP's fatal error at runtime. Extends the existing collect_missing_methods infrastructure (used by both the diagnostic and the "Implement missing methods" code action) with a new collect_abstract_from_used_traits pass. Also fixes an early-return that previously skipped classes with only used_traits (no interfaces or parent class).

Incompatible static return type override (Closes #198)

When a parent method declares a static return type (including ?static), child classes must also use static — using self narrows the contract and causes a PHP fatal error at runtime. This diagnostic catches that mistake at edit time. Uses native_return_type rather than the effective docblock type, since PHP only enforces native type hints.

Match arm type checking (Closes #200)

match expressions where a literal arm condition can never match the subject's type under strict comparison (===) now produce a warning. For example, an int literal in a match against a string subject is flagged as unreachable. Uses the Walker trait to collect match expressions from the AST, then resolves the subject type via the forward walker. Handles union and nullable subject types conservatively: a literal is only flagged when it is incompatible with every branch of the union.

Test coverage

• 13 tests for enum diagnostics
• 26 tests for implementation errors (existing suite expanded)
• 9 tests for incompatible override
• 10 tests for match type errors

All pass. Clippy and fmt clean.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 89.64218% with 55 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/diagnostics/match_type_errors.rs	84.81%	29 Missing ⚠️
src/diagnostics/incompatible_override.rs	85.71%	17 Missing ⚠️
src/diagnostics/enum_errors.rs	94.77%	8 Missing ⚠️
src/code_actions/implement_methods.rs	98.11%	1 Missing ⚠️

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