Define compatibility policy for conversions to prior struct shapes

[ben-ranford]

Problem

The bounded declaration comparison in #58 accepts plain defined-struct field additions that preserve existing keyed fields and comparability. Consumers that convert to the former underlying struct shape can still stop compiling.

At PR #88 head 4372762e7e68457f3255eb19b43f43558f2a69ba, an isolated compiled fixture proved:

• Baseline type Options struct { Name string }: struct { Name string }(api.Options{}) compiles.
• Candidate adds Enabled bool: that conversion fails.
• api.Options{Name: "ok"} compiles against both versions.
• The current compiled release-baseline CLI exits 0 and reports status=compatible.

This is distinct from unkeyed literals and promoted selectors (#114). A source inventory alone cannot establish how downstream consumers depend on the old struct shape.

Scope and acceptance

• Define and document the compatibility policy for exported struct shapes and conversions to anonymous or separately defined types.
• Add consumer compilation fixtures for old-shape conversions, pointers, named types and keyed construction controls.
• Identify when a changed exported struct is a documented shape contract and make the release gate reject prohibited additions to that contract.
• Keep additive fields that satisfy the supported consumer contract available; do not silently replace the existing policy with unconditional acceptance or reject all additive fields without an explicit policy decision.
• Preserve the existing anonymous-alias rejection, embedded-selector boundary, comparability checks and unkeyed-literal warning.
• Require explicit compatibility review and affected consumer compilation evidence for current field additions; passing declaration comparison alone is insufficient.
• No new root dependency or minimum-Go increase.

Backlog follow-up outside the v1.1.0 implementation milestone. This issue records an unsupported consumer shape; it does not waive a known breaking change to a promised v1 consumer contract.