feat(server): XDR invoke encoder and envelope codec contract (#96)

[moises-cisneros]

Refs #96

Part of the chain that replaces #119, split after its review. Tracker: #122.

Summary

The XDR encoder builds unsigned invoke envelopes, and EnvelopeCodec defines how the relay builds, parses, compares and verifies one.

Acceptance criteria

The chain as a whole covers #96; this PR carries its slice (see Scope below).

• The app can create and fund a hire on testnet using only prepare… and submitEscrowCall (proven in the docs PR)
• A signed envelope that differs from its preparation in any field is rejected
• A caller can never prepare or submit for a hire that is not theirs
• A submit… call never throws for chain outcomes
• Each preparation has at most one ChainSubmission

Verification evidence

dart analyze --fatal-infos
dart test test/unit/ledger

Notes for reviewers

• Review against the parent branch (feat/96-escrow-relay-01-protocol), not main.
• Generated code, migrations and recorded fixtures are excluded from the authored count.

Chain Context

Field	Value
Chain	Escrow relay prepare and submit endpoints (#96)
Tracker PR	#122
Position	2 of 12
Base	feat/96-escrow-relay-01-protocol
Depends on	#123
Follow-up	#125
Review budget	398 authored (398 total) / 400
Starts at	feat/96-escrow-relay-01-protocol
Ends with	xdr_invoke_encoder.dart, envelope_codec.dart and the encoder test.

Chain Overview

main
 └── #122 Tracker (draft, no merge)
      └── #123 protocol
           └── 📍 #124 xdr-encoder
                └── #125 stellar-codec
                     └── #126 rpc-client
                          └── #127 preparation-store
                               └── #128 hire-lifecycle
                                    └── #129 prepare
                                         └── #130 submit
                                              └── #131 hire-service
                                                   └── #132 endpoint
                                                        └── #133 docs
                                                             └── #134 openspec

Scope

• Includes: xdr_invoke_encoder.dart, envelope_codec.dart and the encoder test.
• Excludes: The stellar_dart implementation and the RPC calls.

Autonomy

• CI is expected to pass for this PR branch
• This PR has one deliverable scope
• This PR can be rolled back without unrelated changes
• Tests, docs, or manual verification cover this unit

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View logs

[moises-cisneros]

Folded into #126 to keep the chain at 4 PRs. The commits are the same, just grouped; this branch stays in the history of #126.

[TOMOKI977]

Approved. This slice is part of the code I reviewed in full on #119 at b0a4adc (all four lenses; see the review there). I checked that the tip of the chain matches b0a4adc except for the CI fix in .env.example and .gitleaks.toml (and main's #112). The follow-ups listed on #119 apply to the chain as a whole.