Make the Drizzle EQL migration rewriter non-destructive and fail closed

[tobyhede]

Part of #778 and the EQL v2 removal.

This is the single safety-policy track for #786 and #811, rather than a collection of parser fixes.

Acceptance criteria:

• Generated migrations never contain an executable DROP COLUMN or RENAME COLUMN from the ALTER-to-encrypted rewrite.
• The rewrite may add the encrypted column, but it must preserve the source column and direct the user to the staged migration flow.
• When the source type cannot be proven, leave the statement untouched, report a distinct reason, and fail non-zero.
• Keep CLI and wizard implementations behaviorally identical through the parity test.
• Cover partial writes (Drizzle sweep: partial-write-then-throw is reported as unverified, not destructive #786), dollar-quoted DDL (Migration rewriter: dollar-quoted DDL is invisible to the corpus index, so the already-encrypted guard is bypassed and a live DROP COLUMN is emitted over ciphertext #811), quoted identifiers, strings, CRLF, absent source declarations, and cross-directory histories.
• Treat dollar-quote delimiters lexically: a $tag$ sequence inside a legal unquoted identifier such as price$usd$cents must not open a phantom dollar-quoted body, suppress later ALTER detection, or allow the command to report success. Add matched CLI/wizard regression coverage for tagged and untagged delimiters in identifiers and for a genuine dollar-quoted body.

A prompt or SQL comment is not a sufficient data-loss safeguard.