fix: guard Buffer.from island arguments before bytesNew

[yearth]

Fixes #35.

Summary

• validate the actual lowered source IR before constructing bytesNew for Buffer.from
• fall back to the existing SC2020 unsupported-shape diagnostic for island-backed Uint8Array and number[] values
• add a dynamic regression fixture for both container branches and record its order-parity baseline

This preserves the currently supported Buffer.from surface; it only prevents an invalid jsval source from reaching bytesNew.

Testing

• pnpm -r build
• pnpm lint
• pnpm exec vitest run tests/harness/diagnostics.test.ts (101 passed)
• pnpm exec vitest run packages/compiler/test/ts7/order-parity.test.ts (23 passed)
• SCRIPTC_TEST_WORKERS=4 pnpm test (3348 passed, 54 skipped)
• SCRIPTC_TEST_WORKERS=4 SCRIPTC_SAN=1 pnpm test (3345 passed, 54 skipped, 3 parallel timeouts; all three passed in a single-worker targeted rerun)

[vercel]

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[yearth]

Refreshed this PR onto current main (a737fde); the patch is unchanged in scope, and main still does not contain an equivalent guard.

Validation on the refreshed commit (c09bdb5):

• pnpm -r build
• pnpm lint
• diagnostics: 110/110 passed
• order parity: 24/24 passed
• SCRIPTC_TEST_WORKERS=4 pnpm test: 75 files passed, 3582 tests passed, 161 skipped
• SCRIPTC_TEST_WORKERS=4 SCRIPTC_SAN=1 pnpm test: 75 files passed, 3582 tests passed, 161 skipped

pnpm test:sandbox could not start locally because SCRIPTC_SANDBOX_IMAGE is unavailable, so I used the repository-prescribed local fallback.

[yearth]

@ctate Hi! Just a gentle follow-up on this PR when you have a chance. I refreshed it onto the current main and reran build, lint, diagnostics, order parity, and both the plain and sanitizer full suites; everything passed. The change remains the same focused fix for #35, without expanding the supported Buffer.from surface.

Happy to adjust anything if you would prefer a different direction. Thanks for your time!