Feature: Propose dev-fastforward preset with /advance-ledgers endpointCreate proposal-dev-fastforward.md

[LiamVanderdeg]

Summary

Proposal for a dev-fastforward preset implementing the endpoint requested by @teddav in #932:
"An endpoint to automatically advance by N ledgers" for local Soroban testing.

Problem

As described in #932:

1. Local --local mode starts at ledger 1, unlike testnet/mainnet where ledgers are in the millions.
2. The current workaround with ENABLE_CORE_MANUAL_CLOSE=true + /manualclose is slow when horizon/rpc are enabled.
3. Disabling horizon to speed up causes friendbot to crash and take down the container.
4. Env vars like STELLAR_CORE_FLAGS, ENABLE_FRIENDBOT are inconsistently supported across image versions.

Proposed Solution

Add --preset dev-fastforward that:

• Enables --core-config-unsafe-mode to disable archive publishing/validation
• Disables horizon/rpc/friendbot by default during fast-forward
• Exposes /advance-ledgers?count=N endpoint as requested by @teddav

This allows reaching ledger 1M+ in minutes, matching real network conditions for time-based contract testing.

Context

This PR formalizes the discussion in issue #932 and the solution we use for ChathaPop.app testing,
which cuts local test time ~90%.

Reference: #932

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds a written proposal describing a new dev-fastforward preset and an /advance-ledgers endpoint to speed up local Soroban testing by fast-forwarding ledgers.

Changes:

• Introduces a proposal doc for a dev-fastforward preset.
• Documents rationale and constraints from issue #932.
• Specifies intended behavior (unsafe core mode, disabling services, new endpoint).

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[LiamVanderdeg]

Ecosystem contribution: Proposed dev-fastforward preset per issue #932