docs: backfill CLAUDE.md, READMEs, stub docs, and component doxygen

[zeroecco]

Summary

• CLAUDE.md (repo root): codebase orientation — project purpose, Bazel build commands, directory layout, subsystem descriptions, language quick reference, links to existing docs
• zirgen/dsl/README.md: full DSL front-end pipeline description — lexer token table, parser rule table, AST node catalogue, and lowering overview
• zirgen/Dialect/README.md: MLIR dialect hierarchy (ZHL → ZHLT → ZStruct → Zll → BigInt / IOP / R1CS) with per-dialect purpose and compilation flow diagram
• zirgen/docs/99_Arrays_and_Loops.md: array literals, ranges, for loops, subscripts, reduce, generic array components, in-loop constraints — with working examples drawn from test suite
• zirgen/docs/99_Backs.md: @N back-reference syntax, forward declarations, component/parameter/array backs, cross-cycle constraints, cycle-boundary behaviour — with fibonacci and counter examples
• zirgen/docs/99_Externs.md: extern declaration syntax, nondeterminism contract, aggregate return types, built-in externs table, host-side implementation notes, security considerations
• 9 component headers (bits.h, bytes.h, ram.h, u32.h, fpext.h, iszero.h, onehot.h, plonk.h, mux.h): doxygen block comments on all public classes and key structs describing purpose, fields, and intended use

No source code was modified; all changes are documentation only.

Test plan

• Verify new Markdown files render correctly on GitHub
• Confirm doxygen comments parse cleanly (IDE hover or doxygen tool)
• Build the compiler to confirm no header regressions: bazel build //zirgen/Main:zirgen

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