feat: support optional position param in string includes() and endswith()

[cs01]

Summary

• string.includes(search, position) now accepts optional position parameter per JS spec (was rejecting with error)
• string.endsWith(search, endPosition) now accepts optional endPosition parameter per JS spec (was rejecting with error)
• Position is clamped to valid range (negative → 0, past end → string length)

Test plan

• tests/fixtures/strings/string-includes-position.ts — position=0, mid-string, past end, negative, non-existent
• tests/fixtures/strings/string-endswith-position.ts — substring match at position, suffix longer than position, clamp past end, negative
• npm run verify:quick passes (tests + Stage 1 self-hosting)

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