Skip editor validation when the parse result can't have changed

[Edwinexd]

Addresses part of #19 (battery usage).

The live "validate on change" effect ran on every keystroke, calling validateStatements (SQLite prepares/parses via iterateStatements, Postgres runs EXPLAIN) and, in RA mode, raToSQL → getColumnNames → db.prepare(). No debounce, no memoization. That per-keystroke parsing is the main CPU/battery cost while typing.

Instead of a blanket timer, skip the engine call only when the result provably can't have changed since the last validation:

• Exact repeat: same query string as last validation (effect re-fired for an unrelated dependency) reuses the previous result.
• Syntax error + append: a real syntax error fails at the first bad token, so appending further whitespace-separated tokens after an already-validated syntax error leaves that earlier failure unchanged. The whitespace check prevents appended characters from merging into the failing token. Excludes incomplete-input errors (appending may complete them) and semantic errors like missing table/column (appending changes name resolution).

Error classification covers both engines:

	SQLite	Postgres (EXPLAIN)
incomplete	incomplete input	syntax error at end of input
syntax	near "X": syntax error	syntax error at or near "X"
semantic	no such table: X	relation "x" does not exist

Persistence (localStorage + written-questions tracking) is split into its own effect so it stays immediate; only the engine validation is skipped.

RA mode keeps only the exact-repeat guard since its error shapes don't fit the append invariant.

Existing suite passes (249 tests), tsc and eslint clean.

[cloudflare-workers-and-pages]

Deploying sql-validator with    Cloudflare Pages

Latest commit:	e471245
Status:	✅  Deploy successful!
Preview URL:	https://11180a35.dsv-db-sql-validator.pages.dev
Branch Preview URL:	https://fix-issue-19-battery-usage.dsv-db-sql-validator.pages.dev

View logs