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