docs: prepare v1.4.0 evolution API release

[johndauphine]

Summary

• document the merged public schema-evolution DDL API, capability limits, typed unsupported behavior, and batch execution contract
• add the complete public-evolution entry to the Unreleased changelog for the upcoming compatible v1.4.0 release
• correct the published minimum Go requirement for downstream consumers

Release scope

This PR prepares release documentation only. It does not tag or publish v1.4.0.

Validation

• go test ./...
• go build -trimpath -o /private/tmp/smt-v1.4.0-docs ./cmd/smt
• git diff --check

[johndauphine]

@copilot review

[Copilot]

Pull request overview

This PR updates release documentation for the newly merged public schema package surface, specifically adding/expanding documentation for the typed schema-evolution DDL API and recording the change in the Unreleased changelog for the upcoming v1.4.0 release.

Changes:

• Documented the public schema-evolution API (typed operations, capability reporting, and batch execution contract) in docs/public-ddl-api.md.
• Added an Unreleased changelog entry describing the public schema-evolution DDL API and its compatibility characteristics.
• Corrected/clarified downstream minimum Go version expectations for consuming the module.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
docs/public-ddl-api.md	Adds a new “Schema evolution” section describing the public evolution API, its typed unsupported behavior, and batch execution contract.
CHANGELOG.md	Adds an Unreleased “Added” entry announcing the public schema-evolution DDL API and noting the renderer version bump.

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[Copilot]

@copilot review

Addressed both issues from the review in commit docs: fix HasDefault wording and add missing #247 link definition:

1. docs/public-ddl-api.md — Rewrote the HasDefault description to clarify that it signals the presence of a DEFAULT clause (not specifically an empty-string default), which avoids misleading callers about which fields to use when distinguishing "no default" vs "default present but empty expression".

2. CHANGELOG.md — Added the missing [#247]: https://github.com/johndauphine/smt/issues/247 reference link definition at the bottom so the link renders correctly.

[johndauphine]

@copilot review

[Copilot]

@copilot review

The PR is clean. Both issues from the earlier automated review are resolved in a78d492:

• HasDefault wording in docs/public-ddl-api.md correctly describes it as signalling the presence of a DEFAULT clause (not specifically an empty-string default).
• [#247] reference link definition is present in CHANGELOG.md so the link renders.

No new issues found.