feat(external-call): prepared-transaction codec for recorded results - #21

…l-asset#549)

## Summary

This is PR 1 of 8 in the external-call runtime-integration series tracked in digital-asset#513, building on digital-asset#537 (now merged to `main`).

The external-call stack (digital-asset#513): digital-asset#506 added the transaction-side representation for recording external-call results; digital-asset#514 added the LF `EXTERNAL_CALL` builtin surface; digital-asset#518 wired it into Speedy and the LF engine; digital-asset#522 added transaction protobuf encoding/decoding; digital-asset#526 added Canton protocol serialization for recorded results; digital-asset#537 added the participant-side extension-service client. digital-asset#541 implemented the remaining runtime integration as one PR; this series splits digital-asset#541 into 8 independently reviewable PRs (each < 1000 lines) and supersedes it.

This PR lays the protocol and transaction-hash foundation the rest of the series builds on: the Canton protocol fields that carry recorded external-call results, and the `HashingSchemeVersion` V4 hashing that covers them.

## Scope

This PR adds:

- `ViewParticipantData.externalCallResults` and the supporting protocol additions (`participant_transaction.proto`, `SubmitterMetadata`, `ExternalAuthorization`, `LocalRejectError`), including the `ImmArray → Seq` migration of the recorded-results collection across the base/common/participant transaction-tree construction sites
- `HashingSchemeVersion` V4 (dev-gated) and the v4 node/transaction hashers that fold recorded external-call result payloads into the transaction hash, with the payloads surfaced in the `HashTracer` output
- the `HASHING_SCHEME_VERSION_V4` Ledger API enum value and the matching `SubmitRequestValidator` case (required for `-Werror` match exhaustiveness once the enum exists)
- the daml-lf `transaction.proto` field for recorded results
- the regenerated protobuf snapshot and `openapi.yaml` (V4 enum), plus hash tests, generators and docs

## Out of scope

The prepared-transaction codec (PR 2), command-execution wiring (PR 3), engine replay (PR 4), the extension-service runtime (PR 5+), consistency checking, routing and validation (PR 6–8). No LF semantics change. The feature is `ProtocolVersion.dev`-gated, so this PR is dormant on released protocol versions.

## Hashing Boundary

Recorded external-call result payloads are Canton protocol data: after submission they are carried in `ViewParticipantData.externalCallResults` and protected by the Canton view signature. V4 hashing folds them into the Canton transaction hash on that basis. (Their exclusion from the LF/prepared node hash is covered in PR 2.)

## Stacking & review

PR 1 of an 8-PR series; it is the base, so its diff against `main` is already the clean per-PR change. The series is stacked, so PRs 2–8 should be reviewed/merged in order (each rebased onto `main` as the previous lands). Dependency notes are in each PR.

Refs digital-asset#513.

* [main] Update 2026-07-02.15

Reference commit: 4beb999

* Updated docker image for Canton builds

…ults