fix: avoid throwing in warn mode for invalid types

[robinchen0127-star]

Problem Summary

warn: true still throws for invalid $Shape(...) constructions because $ShapeType.errors() raises an invariant instead of yielding a validation error. That makes t.check() / t.warn() escape instead of logging a warning.

Solution Approach

• replace the $ShapeType.errors() invariant with a yielded validation error tuple
• keep assert behavior intact by covering the same invalid construction under assert mode
• add a regression test proving warn mode logs instead of throwing

Test Evidence

cd packages/flow-runtime
npx mocha --grep "warn instead|assert mode"
# 2 passing

npx mocha
# 170 passing

Closes #113

[robinchen0127-star]

Quick follow-up: this PR is ready for review.

It keeps assert-mode behavior intact, replaces the warn-mode throw with a validation error, and includes the regression coverage noted in the PR body. Local targeted and full mocha runs passed on my side.

Happy to adjust anything if you'd like the fix narrowed further.