Hardcoded `dialect: "snowflake"` across altimate-core tools

## Problem

The altimate-core tools don't expose a `dialect` parameter — they always target Snowflake internally. The `sql_quality` telemetry event supports an optional `dialect` field, but only the sql-* tools (analyze, optimize, schema-diff) populate it since they accept dialect as a user parameter.

### Tools missing dialect support

- `altimate-core-validate.ts`
- `altimate-core-fix.ts`
- `altimate-core-correct.ts`
- `altimate-core-semantics.ts`
- `altimate-core-equivalence.ts`
- `altimate-core-policy.ts`
- `altimate-core-check.ts`
- `impact-analysis.ts`

### Tools that already support dialect

- `sql-analyze.ts` — `dialect` parameter, passed to dispatcher and telemetry
- `sql-optimize.ts` — same
- `schema-diff.ts` — same

### Proposed fix

1. Add an optional `dialect` parameter to all altimate-core tools (defaulting to `"snowflake"`)
2. Pass the dialect through to the Dispatcher call
3. Include `dialect` in metadata so telemetry picks it up

This enables per-dialect telemetry segmentation and prepares these tools for multi-dialect support (postgres, bigquery, duckdb, etc.).

### Context

Identified during PR #446 (`sql_quality` telemetry). Rather than hardcoding `dialect: "snowflake"`, we made the telemetry field optional — tools that don't set it simply omit it from the event.