Explain: let plugin drivers return raw explain output parsed by @tabularis/explain (SQL Server SHOWPLAN)

[debba]

Context

For built-in drivers, explain_query returns ExplainQueryOutput::Raw { raw: RawExplainOutput } — a payload plus a format tag — and the parsing lives in the @tabularis/explain TypeScript package (parseRawExplain).

For plugin drivers, the host wraps the JSON-RPC result as ExplainQueryOutput::Plan { plan } and passes it to the frontend untouched (src-tauri/src/plugins/driver.rs). A plugin must therefore ship its own parser and emit the visual-plan model directly: the SQL Server plugin currently parses SHOWPLAN_XML in Rust (src/driver/showplan.rs) and returns the parsed tree.

Goal

Make plugin explain output flow through @tabularis/explain the same way built-in drivers do:

1. Add the SQL Server SHOWPLAN XML parser to @tabularis/explain (a complete implementation already exists in PR feat(sqlserver): add built-in SQL Server driver tabularis#560: packages/explain/src/parsers/sqlserver.ts + tests, format tag sqlserver-showplan-xml).
2. Extend the plugin protocol so a plugin's explain_query can return a RawExplainOutput-shaped result ({ engine, format, payload, original_query }) that the host forwards as ExplainQueryOutput::Raw, letting parseRawExplain handle it in the frontend. Plugins that already return a parsed plan keep working (Plan pass-through stays the fallback).
3. Switch the SQL Server plugin to return the raw XML and drop its in-process parser.

Why

• One parser, one place: plan-rendering fixes and new metrics land in @tabularis/explain without re-releasing plugins.
• explain.tabularis.dev consumes @tabularis/explain, so it gains direct SQL Server compatibility — paste a SHOWPLAN_XML document and get the visual plan, with no plugin involved.
• Keeps plugins thin: they hand over what the engine produced, tagged with its format, exactly like built-in drivers.

Notes

• The parser from PR feat(sqlserver): add built-in SQL Server driver tabularis#560 sums per-thread ActualRows/ActualExecutions and takes the max ActualElapsedms from RunTimeInformation, so STATISTICS XML (Analyze) is covered too.
• Registry/versioning: the manifest could declare an explain_format (or the result itself is self-describing via the format tag) so older hosts keep treating plugin results as pre-parsed plans.