Repo hygiene: delete dead code, retire prg2bin.ts, move the src/main.ts scratch pad

[kameronbrooks]

Plan items #26, #32, #42–#46, #50 — repo hygiene, no behaviour change:

• #26 src/nsengine/prg2bin.ts — unfinished and unused (no length prefix, mismatched const/instruction offsets, console.logs). Superseded by src/wasm/bytecode.ts. Delete, or rewrite against the WASM serialiser.
• #32 The SQRT token is unreachable in the tokenizer (the // comment branch shadows it) and the powRoot handling is dead. Pick a different spelling (√?) or remove it.
• #42 src/utilities/state_machine.ts — never imported, and its compile() compares a token to a PatternElement object. Delete or finish.
• #43 Generator in collection_iterator.ts — implements ICollectionIterator, never instantiated.
• #44 ast.compileTimeSolve() / ast.isKnownAtCompileTime() — superseded, no callers.
• #45 ExternalFunction and Interface classes in executor.ts — unused.
• #46 IMPLICIT_CONVERSION_MAP, OP_MAP / searchOpMap in program.ts — replaced by the DType operation tables.
• #50 src/main.ts is a scratch pad shipped in src/ — move to examples/ or scripts/dev.ts.