feat: showcase library effects, structs, and termination checking

[magnus-madsen]

Adds three new feature sections to the front page, each with a code example based on the Programming Flix book:

• Library Effects (after Algebraic Effects): Clock/Env/FileRead/Logger example with default handlers; prose covers the effect roster and middleware (retries, circuit breakers, sandboxed/in-memory filesystem)
• Mutable Structs (after Region-based Local Mutation): Person struct with a mut field and companion module; prose covers mut fields, unboxed fields, and encapsulation
• Termination Checking (after Seamless Java Interoperability): @Terminates tree-recursion example; prose explains the structural recursion check

Also:

• Adds "automatic trait derivation", "default effect handlers", and "string interpolation" to the Complete Feature List (columns stay balanced at 9 items each)
• Adds a "Test Framework" row to the Tooling Comparison Table (flix / OUnit, alcotest / HUnit, tasty)
• Restores strict code/text alternation across all showcase rows (including the hero → ADT boundary) by flipping the sides of the rows from ADT through Mutable Structs; the only prose change needed was "program on the right" → "left" in Algebraic Effects

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