Add IR to compiler; validate in smart constructors

[hhugo]

Summary

• Introduce Ir.t, a pure intermediate representation for sedlex patterns that captures regexp structure and named captures before tag allocation. Lives in src/compiler/ir.ml with no ppxlib dependency.
• Add Sedlex.compile_ir : Ir.t array -> compiled_ir to the compiler library. It handles tag allocation (Start_plus/End_minus optimizations), discriminator insertion for or-patterns, and DFA construction.
• The PPX becomes a thin translator from OCaml pattern AST to Ir.t via ir_of_pattern, delegating all optimization decisions to the compiler.
• Flatten Alt from binary to n-ary (Alt of t list), simplifying discriminator assignment to a single pass and removing the reusable_cell heuristic.
• Move structural validation into IR smart constructors returning (t, string) result: capture checks shadowing, alt checks name consistency across branches, star/plus/rep reject inner captures. The PPX uses a local unwrap helper to bridge results to ppxlib error reporting.
• Use SSet instead of string list for capture_names.
• Improve error messages: bare operators (Star, Plus, Opt, etc.) without arguments now produce specific errors instead of falling through to "not a valid regexp"; unknown constructors produce "unknown sedlex operator X".

Motivation

The PPX previously interleaved pattern parsing with tag allocation decisions (bind vs bind_start_only vs bind_end_only), discriminator handling, and fixed_length computation. This made it impossible to test the compilation pipeline without ppxlib, and spread compiler concerns across two packages.

With the IR, tests can construct Ir.t values directly and exercise the full compile_ir path. Future optimizations (tag delay, register allocation) can be added to the compiler without touching the PPX.

Test plan

• dune build succeeds
• dune runtest passes — all existing expect tests produce identical output (modulo disc cell renumbering from the flat Alt change)
• Error messages preserved for all validation cases (captures inside Star/Plus/Rep/Opt/Compl/Sub/Intersect, mismatched or-pattern names)

🤖 Generated with Claude Code

[hhugo]

@toots, I think you'll like this PR as well. It will help with #194 quite a bit.