Consolidate the mint/burn ExtraArgs context builder (V1 cleanup + V2 prep)

[gyorgybalazsi]

Summary

The Token Standard mint/burn ExtraArgs / ChoiceContext map is hand-written three times, all untyped/string-keyed, and the copies must stay byte-identical. CIP-112 (Token Standard V2) will add a parallel set of V2-namespaced keys (dual-version support), roughly doubling each copy.

Consolidate now, as a V1-only refactor (no V2 dependency), so the key set + context types are defined once per language and adding the V2 keys later is a one-place change.

Prep, not the V2 migration itself. Safe to land before the Token Standard V2 API freeze (Splice 0.6.11, ~Jul 2 2026). Tracked in the CIP-112 cBTC scoping doc §9.3 (shared-builder note) and §5.5 (pre-Jul-2 prep).

Current state — the three copies

Copy	Location	How it builds the context
Rust	cbtc-lib/src/mint_redeem/redeem.rs:419	serde_json::Map + json! literals — untyped serde_json::Value, keys + AV_* tags as string literals
TS app	DLC-link/cbtc-mint-burn → app/src/functions/token-standard.ts:70 (createExtraArgsForMintBurn)	hand-declared AnyValue / ChoiceContext / ExtraArgs interfaces + the same keys
IT helper	DLC-link/dlc-attestor-stack → it/src/canton/helpers/token-standard.ts	third copy; the TS app's header comment says it "matches" this one

All three emit the same keys — utility.digitalasset.com/instrument-configuration, /issuer-credentials (plus optional /app-reward-configuration, /featured-app-right), and splice.lfdecentralizedtrust.org/reason — off the contracts returned by GET /cbtc/v1/token-standard-contracts.

Contrast: the transfer/accept path does not hand-build context — transfer.rs / accept.rs fetch typed choice_context_data from the registry. Only the burn-mint path assembles the map locally. (The Daml side is unaffected: cBTC's choices forward extraArgs opaquely into BurnMintFactory_BurnMint, so DLC-link/canton needs no change.)

Why now

• Drift risk: three untyped copies kept in sync by hand; a mistyped reverse-DNS key or wrong AnyValue tag fails only at exercise time, on-ledger.
• V2 doubles it: dual-version means emitting V2-namespaced keys alongside the V1 keys in every copy.
• Unblocked: pure V1 refactor, no V2 package dependency — can land before the Jul 2 API freeze and shrinks the eventual V2 diff to one place per language.

Proposed scope

• Rust (cbtc-lib): replace the json! context literals in redeem.rs with the codegen'd typed common::*::ExtraArgs types already used by transfer.rs / accept.rs; centralize the burn-mint key set as named constants/types in one module.
• TS: merge the two TS copies — cbtc-mint-burn app + the dlc-attestor-stack IT helper — into a single shared module (one set of AnyValue / ChoiceContext / ExtraArgs types + one keyed builder). Decide where it lives (published TS package vs. single-source-of-truth file).
• Define the context key set once per language as named constants.

Rust and TS can't share one function — "shared" here means one schema / source of truth per language, not one binary.

Out of scope

• Any V2 keys/interfaces (added once the V2 APIs freeze, ~Jul 2 2026).
• The transfer/accept path (already typed + registry-fetched).
• The Daml code in DLC-link/canton (receives extraArgs opaquely; no change).

Acceptance criteria

• One TS module is the sole definition of the mint/burn context types + builder; both the app and the IT helper import it; no duplicated key strings remain.
• redeem.rs uses the typed ExtraArgs types — no json! context literals.
• The utility.digitalasset.com/* + reason keys are defined once per language (named constants), not inlined.
• V1 behavior unchanged across mint, burn (CBTCWithdrawAccount_Withdraw), and the cancel-withdraw mint-back; existing integration tests pass.

Cross-repo

• DLC-link/cbtc-mint-burn — TS app copy (sub-task / PR there).
• DLC-link/dlc-attestor-stack — IT helper copy + the integration tests that gate the refactor.

References

• Scoping doc: CIP-112 (Token Standard V2) — cBTC Support Scoping, §9.3 (shared-builder note) + §5.5 (pre-Jul-2 prep) — https://app.notion.com/p/381636dd0ba581a488acf366a5348131