feat: add tableless rendertree layout

[apstndb]

Summary

• Add a tableless rendering layout for rendertree, exposed through --layout=tableless and the --tableless shortcut.
• Add matching support in the plantree/reference package with a Layout enum-style config option.
• Render tableless output with a one-character | separator, right-align numeric columns, trim table padding, and document the center-alignment limitation.
• Update CLI and reference docs plus focused tests and golden output coverage.

Validation

• go test ./...
• go test ./asciitable
• go test ./cmd/rendertree/impl
• go test ./plantree/reference
• git diff --check

[gemini-code-assist]

Code Review

This pull request introduces a new 'tableless' layout option for rendering Spanner query plans, allowing rows to be rendered without an ASCII table grid using a pipe ('|') as a column separator. This feature is implemented in the asciitable package, exposed via the --layout=tableless and --tableless CLI flags in rendertree, and integrated into the plantree/reference API alongside comprehensive tests and documentation. I have no feedback to provide.

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