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

docs/proposal-dev-fastforward.md

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+## 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
+
+## Usage
+Call the endpoint to fast-forward N ledgers from the current state:
+This will fast-forward 1000 ledgers from the current state, not jump to ledger 1000.
+
+## Implementation Notes
+
+**Scope**
+This unsafe mode is intended for local development and testing only. It should not be enabled in production or exposed to public networks.
+
+**Endpoint Protection**
+The `/advance-ledgers?count=N` endpoint is only available when the quickstart container is started with the `UNSAFE_FASTFORWARD=true` environment variable. By default this mode is disabled and the endpoint returns 404. No authentication is implemented; access control relies on keeping the container network-local.
+
+**Limits**
+There is no hard limit on `count=N` in the initial implementation. For large values, the operation may take significant time and consume resources. It is recommended to keep `N` < 100,000 per call for stability.
+
+**Behavior**
+The endpoint blocks the HTTP request until all requested ledgers are advanced. The response is returned only after completion. While advancing, Horizon, RPC, and Friendbot remain disabled to avoid inconsistencies.
+
+**Re-enabling Services**
+Horizon, RPC, and Friendbot are automatically re-enabled on the next container restart without `UNSAFE_FASTFORWARD=true`. There is no hot-reload mechanism in this proposal.