docs(iOS): Xcode 15 + iOS targets & build steps

[captainvredina]

Summary

• Update Swift/iOS README to Xcode 15
• Add rustup targets & iOS/simulator build steps
• Add XCFramework integration + troubleshooting

Why

• Existing instructions reference older Xcode and omit target setup, causing build failures.

Verification

• Steps are reproducible with Xcode 15 + latest rustup; CI/docs-only change.

Closes: #759 (Bounty – 50 SigUSD)

[coveralls]

Pull Request Test Coverage Report for Build 16540015439

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.05%) to 78.503%

---

Totals
Change from base Build 15758737784:	0.05%
Covered Lines:	11967
Relevant Lines:	15244

---

💛 - Coveralls