Update toolchains for Xcode 26.6 and Swift 6.3.3

[Kyle-Ye]

Summary

• Move package manifests and CI to Swift 6.3 / Swift 6.3.3 and Xcode 26.6.
• Remove Swift 6.2 index-store workarounds from SwiftPM, Xcode, and Tuist paths.
• Update the recommended toolchain documentation.

Impact

This keeps package metadata and CI aligned with the Swift 6.3.3 / Xcode 26.6 toolchain.

Validation

Changed package and workflow manifests parse successfully with Swift 6.3.3 and Xcode 26.6.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 32.17%. Comparing base (49a4009) to head (0f880cd).

Additional details and impacted files

@@ Coverage Diff @@## main #234 +/- ##
=======================================
Coverage 32.17% 32.17% =======================================
Files 71 71 Lines 2601 2601 =======================================
Hits 837 837 Misses 1764 1764 

Flag	Coverage Δ
ubuntu	12.03% <ø> (ø)

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