fix(examples): euler-multichain — anchor base/arbitrum demo windows at first vault creation

[abernatskiy]

Summary

• The euler-multichain example's base and arbitrum demo windows were anchored at the factory-deploy block (base 18_000_000, arbitrum 255_000_000), which is far earlier than the first vault. The ~DEMO_SPAN window landed in an empty pre-vault range, so both chains indexed 0 blocks / 0 vaults — and any run through scripts/examples-e2e.mjs failed its per-chain inserted.blocks > 0 gate (only mainnet had data).
• Arbitrum was doubly off: its GenericFactory emits non-ProxyCreated events well before the first vault, so even its first factory activity (block 300,690,953) wasn't a child creation.
• Fix: anchor each chain's demo window at its first EVault ProxyCreated, verified against the Portal dataset (base 36_016_090, arbitrum 317_852_641). PONDER_FULL=1 still backfills from the factory-deploy block. Mirrors the existing DEMO/FULL-start split already used for mainnet.

Test plan

• biome check clean on the changed config.
• Ran the example end-to-end (built @subsquid/ponder@0.16.9-sqd.1, dedicated Portal, PONDER_DEMO_SPAN=200000): all three chains now discover vaults and clear the per-chain inserted.blocks > 0 gate —
  • mainnet: 39 vaults (648 blocks) — unchanged
  • base: 3 vaults (3 blocks) — was 0
  • arbitrum: 5 vaults (1 block) — was 0
• Note: on a fresh clone the example still uses the free public Portal (unchanged); the fix is purely the window constants.

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