feat(server): escrow relay foundations: protocol, config, envelope codec and RPC (#96)

[moises-cisneros]

Refs #96

Part of the chain that replaces #119, split after its review. Tracker: #122. Regrouped from a first 12 PR split (#123, #124, #125 were folded into this one).

Summary

Everything the relay stands on, with no behavior yet: the protocol models and the escrow_preparation table (with generated code and migration), the relay configuration without defaults (ADR-0005 D3), the XDR encoder, the EnvelopeCodec contract with its stellar_dart implementation (field by field comparison and ed25519 verification, backed by a spike on recorded testnet envelopes), and the Soroban RPC calls (sequence, simulate, send, get transaction). Also the CI fixes from the review of #119: the three variables in .env.example and narrow gitleaks allowlists.

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/protocol test/spike test/ledger test/unit/ledger
bash scripts/tests/env-inventory.test.sh   -> 71 passed
gitleaks over the whole tree                -> no leaks

Notes for reviewers

• Review against the parent branch (feat/96-escrow-relay), not main. The commits are ordered to be read one by one.
• Over the 400 line budget: 2329 authored lines, mostly tests of code that moves funds. Splitting further cuts the unit apart, so it needs size:exception.
• 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	1 of 4
Base	feat/96-escrow-relay
Depends on	#122 (tracker)
Follow-up	#129
Review budget	2329 authored (11660 total) / 400
Starts at	feat/96-escrow-relay
Ends with	Protocol models, migration, HireRelayConfig, SessionWallet, XDR encoder, envelope codec, RPC calls, .env.example, .gitleaks.toml, with their tests.

Chain Overview

main
 └── #122 Tracker (draft, no merge)
      └── 📍 #126 escrow relay foundations: protocol, config, envelope codec and RPC
           └── #129 persist preparations and prepare unsigned escrow envelopes
                └── #132 submit signed escrow calls and expose the relay in the hire endpoint
                     └── #134 escrow relay API, ADRs, README, testnet e2e and OpenSpec archive

Scope

• Includes: Protocol models, migration, HireRelayConfig, SessionWallet, XDR encoder, envelope codec, RPC calls, .env.example, .gitleaks.toml, with their tests.
• Excludes: Stores, services and the endpoint.

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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[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.