fix: refresh all network genesis hashes (0.12.1)

[Imod7]

Description

All three built-in network genesis hashes pointed at chains that no longer exist. This updates them to the values the chains return today and releases 0.12.1.

Constant	Was	Now
PASEO_ASSET_HUB	0xbf0488...ef19f	0x23e730eb1c6fecae09c917439a5038cb6122d0d48980e8b9bbf0ff56f94a2ca6
PREVIEWNET	0x477dd8...12525	0x8c27ddf678c2ae9bef0efebfc485a9309f3d735c6d3fbb8d947afc3ace0e80f4
PREVIEWNET_ASSET_HUB	0x860d75...c7867c	0x4d11c803cc6921429e3876638977ad006ea1bba8cd3976a0bca2f164e7026210

Changes

File	Change	Impact
src/networks.ts	The three genesisHash literals. rpcUrls are unchanged, all three endpoints respond.	The host now recognises the chains products actually connect to.
test/integration.spec.ts	The chain-feature test repeated the Paseo hash as a literal. It now reads PASEO_ASSET_HUB.genesisHash.	The test would have broken on this change. The next reset is one edit instead of two.
package.json	0.12.0 to 0.12.1.	Patch, no API change.
CHANGELOG.md, forum-post.md	0.12.1 entries.	Required by the repo's release checklist in CLAUDE.md.

README.md needs no change, its network table lists export names and no hashes.

Why these changes

The host routes each connection request to a network by genesis hash. A stale pin means the host does not recognise the chain a product asks for, so the product hangs at connection-status: "connecting" until the test times out. The RPC endpoint is reachable and nothing looks wrong in the logs, which makes this expensive to diagnose.

This blocks paritytech/product-sdk#297, where four demos fail boot.spec.ts for exactly this reason.

Testing

pnpm install
pnpm clean && pnpm build
pnpm typecheck
pnpm test
pnpm test:integration

Verify the pins against the live chains:

for u in https://paseo-asset-hub-next-rpc.polkadot.io \
         https://previewnet.substrate.dev/relay/alice \
         https://previewnet.substrate.dev/asset-hub; do
  curl -s -H 'Content-Type: application/json' \
    -d '{"id":1,"jsonrpc":"2.0","method":"chain_getBlockHash","params":[0]}' "$u"; echo
done