Add support for GEOMETRY/SPATIAL indexes

[morgo]

A Pull Request should be associated with an Issue.

We wish to have discussions in Issues. A single issue may be targeted by multiple PRs.
If you're offering a new feature or fixing anything, we'd like to know beforehand in Issues,
and potentially we'll be able to point development in a particular direction.
Further notes in https://github.com/block/spirit/blob/main/.github/CONTRIBUTING.md

Fixes #686

.. but it uses a go.mod replace to do so. We need to wait for the TiDB PR to merge, and hope it adds SPATIAL index support too. We can followup with a PR if it does not.

[Copilot]

Pull request overview

This PR aims to enable Spirit to handle MySQL GEOMETRY columns and SPATIAL indexes (per #686), primarily by updating parser behavior (via a go.mod replacement) and adding integration/unit tests that exercise DDL parsing and replication/copy pipelines with spatial data.

Changes:

• Add statement parsing test coverage for rewriting CREATE SPATIAL INDEX into ALTER TABLE ... ADD SPATIAL INDEX.
• Add buffered replication and buffered copier integration tests to validate GEOMETRY values round-trip correctly.
• Add a migration integration test that runs an ALTER TABLE adding a generated GEOMETRY column and a SPATIAL index, plus update Go module dependencies (including a replace to a forked TiDB parser).

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
pkg/statement/statement_test.go	Adds coverage for CREATE SPATIAL INDEX rewrite behavior.
pkg/repl/subscription_buffered_test.go	Adds integration test ensuring buffered-map replication preserves GEOMETRY row images and applies correctly.
pkg/migration/migration_test.go	Adds migration test for combined generated GEOMETRY column + SPATIAL index DDL.
pkg/copier/buffered_test.go	Adds buffered copier test verifying GEOMETRY values copy correctly (incl. SPATIAL index presence).
go.mod	Adds indirect dependency and replace to a forked TiDB parser module.
go.sum	Updates sums for new/changed module dependencies.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

The committed replace pins github.com/pingcap/tidb/pkg/parser to a personal fork. This makes builds depend on a non-upstream module path/commit and can break reproducibility and supply-chain expectations. Consider waiting for the upstream TiDB parser change to be released and then updating the require version (removing the replace), or alternatively vendor/fork under an org-controlled module path with a clear upgrade plan and tracking issue.

Suggested change

	replace github.com/pingcap/tidb/pkg/parser => github.com/morgo/tidb/pkg/parser v0.0.0-20260409131709-d591801319ba