Fix #16: add strict mode to reject unknown ViewDefinition select directives

[piotrszul]

Fixes #16.

Problem

Unknown directives on a select element (e.g. repeat, which flatquack doesn't implement) were silently ignored. validateElement only inspected keys it recognized, so an element with an unknown key plus a column array passed as an ordinary column block, and parseNode dropped the directive entirely — producing a valid-but-wrong query (one all-NULL row per resource) instead of failing.

Fix

Add a strict option, default false (current lenient behavior unchanged), threaded parseVd → validateVd, plus a --strict CLI flag.

When strict is on, validateElement rejects any key on a select element that isn't a recognized directive (column/select/forEach/forEachOrNull/unionAll), naming the offender:

unsupported select directive: 'repeat'

The root ViewDefinition keys (resource/name/where/…) are unaffected — the strict check is applied only to select elements (the root validateElement(vd) call is not flagged as a select element).

Verification

End-to-end via the CLI on the issue's repeat view:

• default: generates SQL, exit 0 (lenient, unchanged)
• --strict: error: unsupported select directive: 'repeat'

Tests (tests/view.test.js)

• strict mode rejects an unknown directive (repeat)
• non-strict (default) ignores it
• strict accepts a view using only known directives

All 194 tests pass. tests/spec-tests/ (reference fixtures) untouched.

Implementing repeat itself remains a separate, larger enhancement; this PR only makes the unsupported subset fail loudly when callers opt in.