Graceful degradation when a referenced builder has no copy function #310

Sub-task of #301.

Goal

When a referenced builder has no dedicated copy function (B(T) ctor or create(T)/of(T) factory), the generated consumer builder seeds the empty builder itself: create it via the builder's empty-instantiation path, then call each field function to copy the source value's properties.

How it works

• The seeded path is emitted as a block lambda, e.g. source -> { B b = B.create(); b.name(source.name()); ...; return b; } — a method reference cannot express a sequence, so BuilderInstantiation gains a FieldSeedingCall variant (empty call + ordered (method, accessor) pairs).
• Empty builder required: the builder's own empty-instantiation path (ctor or static factory) is reused — already part of the contract.
• Field function detection: per readable property of T, find an accessible single-param method on B whose name matches a ranked convention — name(v) first (Lombok / Immutables / our own style), then setName(v) (JavaBeans / FreeBuilder / protobuf). Return type does not matter (fluent or void) since calls are emitted as statements.
• Safety rule: this path only qualifies when every readable property of T has a matching field function. A partially covered property set would silently drop values — the same data-loss issue as an empty-builder fallback. On partial coverage, fall back to the previous plan: skip the helpers that need a seeded path, with a debug log line, and use the plain setter.
• Side effect: FreeBuilder and AutoValue generated shapes resolve through this path (per-property setters, no copy ctor or toBuilder).