Major changes

[VladimirBorodko]

Reworks Codable tree encoding, numeric conversions, and merging, and introduces composable Codec strategies.

• Rewrites encoder and decoder containers, fixing coding paths, index advancement, and conflicting writes. Container conflicts invalidate encoding; direct write errors remain catchable.
• Adds Codec.Box, OptionalBox, Each, Either, and strategy composition for collections, optionals, and Result. Includes strategies for Data, URL, Date, canonical Float representation, and keyed dictionaries.
• Unifies numeric equality and conversions, adds Int128/UInt128 support, widens Float values exactly to Double, and fixes Linux NSDecimalNumber conversion.
• Adds MergePolicy with configurable value and array handling. Nonthrowing merges reuse uniquely owned storage; throwing merges preserve the original tree on failure.
• Clarifies path mutation behavior, adds deterministic allPaths traversal, and expands Swift/Foundation value bridging.
• Adds contract tests, DocC guides, executable documentation examples, and benchmarks.
• Configures Swift 6.3.3 CI on macOS and Linux, including Debug/Release tests, a 95% coverage gate, documentation builds, and manually dispatched releases with API compatibility checks and documentation publishing.

Breaking changes: raises the minimum Swift version from 6.0 to 6.3, removes Enigma.float, and replaces the previous Enigma.Prod* and Enigma.Sum* APIs with composition types in Codec.

Validation: Debug and Release functional tests each pass (182 tests, excluding PerformanceTests); all 19 Python script tests pass. Local validation used Apple Swift 6.4 with full Xcode. The squashed commit preserves the previous branch's file contents exactly.