tw: extract Tw_section.encode — deduplicate build_section before proposal 0001 lands two more sections

[hyperpolymath]

Context

Discovered while reviewing typed-wasm proposal 0001 (#402).

The producer-side and verifier-side currently each carry their own copy of the build_section encoder for typedwasm.ownership:

• lib/codegen.ml:159-177 — producer-side section assembly (called at codegen.ml:2778)
• lib/tw_ownership_section.ml:77-96 — verifier-side reader's equivalent encoder (used by lib/tw_interface.ml)

Both encode the same fixed format today, so this is not a correctness bug right now. But:

Why this matters before proposal 0001 lands

typed-wasm proposal 0001 (hyperpolymath/typed-wasm#34 / PR #76) adds two more producer-neutral sections:

• typedwasm.regions (L2–L6)
• typedwasm.capabilities (L15)

If we replicate the current 2-copy pattern naively, we end up with 3 sections × 2 copies = 6 encoders to keep in sync. Worse, the proposal's regions wire format is richer (field tables, ptr-kind enums, nullability bytes, cardinality) — drift between producer and verifier copies becomes a real correctness risk, not just a maintenance smell.

Proposed prep-work

Extract a single module (working name Tw_section.Encode) that both producer and verifier consume:

module Tw_section : sig
  module Encode : sig
    val ownership : Tw_ownership_section.t -> bytes
    (* Add: regions : ...; capabilities : ... when proposal 0001 lands *)
  end
  module Decode : sig
    val ownership : bytes -> (Tw_ownership_section.t, string) result
    (* Symmetric add: regions, capabilities *)
  end
end

Migrate:

• codegen.ml:2778 to call Tw_section.Encode.ownership instead of the inline encoder.
• tw_interface.ml to call Tw_section.Decode.ownership for read path.
• Delete the duplicate build_section at tw_ownership_section.ml:77-96 (or have it delegate).

Acceptance

• Single canonical encoder per section.
• tw_verify.ml round-trip test still passes (encoder ↔ decoder symmetric).
• No behaviour change at v0 (wire bytes byte-equal pre- and post-refactor).

Scope

• This is prep work; it does not implement proposal 0001.
• Touches OCaml only (lib/); does not affect .affine surface or stdlib.
• Should land before typed-wasm proposal 0001 reaches [accepted] and downstream codegen plumbing starts, otherwise the refactor becomes 3× harder.

References

• Cross-ref: Multi-producer ABI proposal: region/type-schema + capability carrier section (unblocks L2-6 / L15 on emitted wasm) typed-wasm#34, docs(proposals): 0001 multi-producer carrier sections for L2–L6 and L15 (Refs #34) typed-wasm#76, Review request: typed-wasm carrier-section proposal for L2–L6 / L15 enforcement (typed-wasm#76) #402
• Discovered via: ephapax#165 + affinescript#402 paired review (2026-05-30)