Return self from every Expression builder; fold Eq/Gt into Comparison

[MidnightDesign]

Closes #92. Scoped for the 0.3.0 breaking release.

Problem

The fluent builder on Expression is our most-used public @api surface, but nine of its fifteen combinators declared a class-level @internal node as their return type while the other six returned self. So the surface both leaked internal symbols and contradicted itself — eq returned Eq but its sibling neq returned self; gt returned Gt but lt/gte/lte returned self.

Separately, Eq and Gt were BC shims: @internal final subclasses of Comparison that added nothing but a fixed constructor. They only still existed because Expression::eq()/gt() named them as return types before Comparison existed.

Changes

• All fifteen Expression builder combinators now return self. The rationale that was spelled out on not()'s docblock is now the class-level rule, so it moved to the class docblock and the per-method note is gone.
• Expr::eq()/gt() return Comparison and construct through the shared comparison() helper.
• Eq and Gt are removed; they fold into Comparison, which is now final (all six operators are plain instances of it). Comparison::equals() already matches on self, so a former Eq/Gt value and a plain Comparison compare equal — behavior is preserved.
• Docblocks on Comparison and BinaryOperator updated to drop the Eq/Gt references and the "four of six" framing.
• One test's data-provider name-builder discriminated closures from expressions with is_callable(), which only worked because the builders returned final concrete nodes. With self (abstract Expression, possibly-callable via a hypothetical __invoke subclass) PHPStan widened $expr() to mixed. Reordered it to check instanceof Expression first, mirroring the consuming test method.
• Added an UPGRADING note for the break.

BC impact

Structural break — return-type narrowing on public methods plus two class removals. The Roave checker will flag it. Correct for a 0.MINOR bump; frozen after 1.0.0, at which point we'd be stuck publishing Eq/Add/… as permanent public types.

Checks

cs-check, psalm, phpstan, check-deps, phpunit (1152 tests), and infection on changed lines (100% MSI) all pass locally.

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