fix(resolve): bind an extension's own type param from a plain receiver

[Hessesian]

Problem

fun <T : Any> T?.required(field: String): T (Moneta's NullableScope) inferred its return type as the literal "T", so textModel.required("textModel").title searched for title on a class named T and missed.

Root cause (two independent bugs, both hidden by #318's test)

1. The parser stores extension_receiver_type only when the receiver has generics, so a plain T? receiver was empty and CallableInfo had nothing to bind T against. CallableInfo now uses SymbolEntry::declared_extension_receiver() (full type if generic, else base name).
2. A real extension is not a method of its receiver's type, so receiver_based_method never matched and the return came from SignatureDerived, which never substituted from the receiver. It now does for .-calls (callee_receiver_type extracted and shared).

Tests

• callable_info_reports_a_non_generic_type_param_receiver (red before)
• resolve_call_expr_type_substitutes_extension_receiver_for_a_signature_derived_return (red before)
• cargo test (2015), clippy -D warnings, fmt clean

Verification

Literal repro InitUseCase.kt:56:47 in Moneta now resolves (was a Gap); minimal fixture went FilteredCandidate -> resolved. Aggregate Moneta recall is flat (~91.9% member): this is one small slice of the title gap; the rest are separate causes being handled in follow-up PRs.

🤖 Generated with Claude Code

[Copilot]

Copilot review overview

🟡 Changes recommended

Receiver nullability and same-named callable collisions can produce incorrect inferred types.

Review effort: Balanced
Findings: 2 · 1

Open (3)

• Match callable metadata to the selected reachable overload · New
• Preserve receiver nullability during type substitution · New
• Rename abbreviated subst variable to substitution · New

What changed in this PR

Adds generic return-type substitution for extension functions with plain type-parameter receivers.

Changes:

• Adds declared extension-receiver lookup.
• Applies receiver substitution to signature-derived returns.
• Adds regression tests for T?.required().

File	Description
src/​types.rs	Adds declared receiver accessor.
src/​indexer.rs	Uses declared receivers in callable metadata.
src/​indexer/​infer/​chain.rs	Extends receiver-based substitution.
src/​indexer/​infer/​mod_tests.rs	Tests signature-derived substitution.
src/​indexer_tests.rs	Tests plain receiver metadata.

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