feat(tidb): fork catalog/completion/deparse/scope from mysql (PR3a scaffold)

[vsai12]

Summary

Mechanical fork of the four remaining MySQL packages into tidb/, preparing the ground for PR3b (TiDB-specific behavior). Pure import-path rename — no logic changes, no collation flip, no new fields.

Dependency chain that forces these four packages to ship together:

completion → catalog → scope
                    → deparse
deparse → scope
deparse (test-time) → catalog

None of the four compile in isolation, so all four must exist before any tests run.

What changed

• tidb/scope/ (1 file)
• tidb/deparse/ (5 files)
• tidb/catalog/ (120 .go files)
• tidb/completion/ (7 files incl. SCENARIOS)

Why this diff is 57k lines

Previous guidance favors ~500-line PRs. A mechanical fork is the pathological exception — splitting four interdependent packages across 4 PRs produces a chain of unmergeable branches that reviewers can't CI independently, which is harder to review, not easier. Reviewers should verify via CI + spot-check on a handful of files (e.g. tidb/catalog/catalog.go, tidb/deparse/resolver.go), not line-by-line. The behavioral changes land in PR3b, which will be a human-scale diff.

Scope guards honored

• Collation default left at utf8mb4_0900_ai_ci — PR3b flips to utf8mb4_bin
• No new fields on Table/Column/Constraint — PR3b adds them
• MySQL-inherited walkthrough tests kept as-is — PR3b will annotate failures
• mysql/semantic/ not forked (no Go package, metadata only)

Test plan

• go build ./tidb/... clean
• go vet ./tidb/... clean
• go test ./tidb/... -short -count=1 — all passing (ast, catalog, completion, deparse, parser)
• CI green on merge

Follow-up

PR3b (branch feat/tidb-catalog-extensions, branched off this commit) adds:

• Collation flip to utf8mb4_bin
• Table/Column/Constraint TiDB fields
• AUTO_RANDOM / PLACEMENT POLICY / TTL / TIFLASH REPLICA / CLUSTERED wiring
• TiDB completion keywords
• pingcap/tidb:v8.5.5 container tests
• Skip-annotations for MySQL-only inherited tests

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