demo: show_constraints — name/stmt split with automated bzl rename, BNF, HTML, and PNG

[ebembi-crdb]

What this PR demonstrates

This addresses the specific scenario from the review feedback and the SQL Grammar Documentation wiki (Troubleshoot HTML generation):

In diagrams.go, name should exclude the _stmt suffix, while stmt includes it. Each bzl file already has a corresponding entry that includes the _stmt suffix. The automation should update the bzl file entries to also exclude the suffix. In each bzl file, there should only be one entry per diagrams.go entry.

The only file the writer touches

{
    name:   "show_constraints",
    stmt:   "show_constraints_stmt",
    inline: []string{"with_comment"},
},

Pre-conditions (state before this PR)

File	Entry before this PR
docs/generated/sql/bnf/BUILD.bazel	"show_constraints_stmt"
pkg/gen/bnf.bzl	:show_constraints_stmt.bnf
pkg/gen/docs.bzl	:show_constraints_stmt.bnf
pkg/gen/diagrams.bzl	:show_constraints.html ← already correct (SVG genrule always stripped _stmt)

This matches the scenario exactly: three bzl files have the _stmt entry; diagrams.bzl already has the HTML without _stmt because show_constraints_stmt was already defined in source.

What CI does automatically

1. Renames bzl entries — detects name: "show_constraints" (no _stmt) with show_constraints_stmt in BUILD.bazel, bnf.bzl, docs.bzl. Renames all three and commits back to this branch. diagrams.bzl needs no change.

2. Generates BNF — show_constraints.bnf with with_comment inlined. Content differs from the existing show_constraints_stmt.bnf. Appears in the linked generated-diagrams PR.

3. Generates HTML — show_constraints.html rebuilt with WITH COMMENT shown inline instead of as an opaque reference box — a visible content change confirmed in the generated-diagrams PR.

4. Posts PNG preview — CI detects the HTML diff and posts a PNG of the updated diagram as a comment on this PR.

Mirrors the reference PR

Follows the same pattern as cockroachdb#161545 (show_statement_hints): with_comment plays the same role as opt_with_show_hints_options — an inlined sub-rule that produces a meaningful HTML change.

[github-actions]

SQL Diagram Generation Report

✅ Validation Passed

🔄 Diagram changes detected
A PR has been opened to sync these changes: ebembi-crdb/generated-diagrams#30

Changed Diagram Previews

show_constraints

Showing 1 changed diagram(s). Only diagrams that differ from master are shown.

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