smartcontract: add Index account for multicast group code uniqueness

[martinsander00]

Resolves: #3408

Summary

• Introduce an Index account (PDA derived from entity type + lowercased code) that enforces unique multicast group codes onchain and enables O(1) code-to-pubkey lookup
• Integrate Index lifecycle into existing multicast group instructions: create (atomically creates Index), update (renames Index on code change), delete/close (closes Index)
• Add standalone CreateIndex/DeleteIndex instructions (variants 104/105) for migration backfill of existing accounts
• Add Index account type to Go, Python, and TypeScript SDKs

Split from #3256 (1/2). Rust SDK integration in #PENDING.

New derivation workflow: #3256 (comment)

Diff Breakdown

Category	Files	Lines (+/-)	Net
Onchain program	16	+598 / -32	+566
SDKs (Go/Py/TS)	3	+3 / -0	+3
Activator	1	+10 / -1	+9
E2E	1	+0 / -1	-1
Tests	9	+691 / -47	+644
Other	1	+2 / -0	+2
Total	31	+1304 / -81	+1223

~53% tests, ~46% onchain program, ~1% other (activator, e2e, Go/Py/TS SDKs, changelog)

Key files (click to expand)

• smartcontract/programs/doublezero-serviceability/src/state/index.rs — new Index account struct (34 bytes: account_type + pubkey + bump_seed) with Borsh serialization
• smartcontract/programs/doublezero-serviceability/src/processors/index/create.rs — standalone CreateIndex processor for migration backfill
• smartcontract/programs/doublezero-serviceability/src/processors/multicastgroup/update.rs — atomic Index rename when multicast group code changes (close old, create new)
• smartcontract/programs/doublezero-serviceability/src/processors/multicastgroup/create.rs — creates Index account alongside multicast group
• smartcontract/programs/doublezero-serviceability/src/processors/index/delete.rs — standalone DeleteIndex processor for cleanup
• smartcontract/programs/doublezero-serviceability/src/pda.rs — get_index_pda() with case-insensitive seed derivation
• sdk/serviceability/go/state.go, sdk/serviceability/python/serviceability/state.py, sdk/serviceability/typescript/serviceability/state.ts — add Index account type

Testing Verification

• All existing integration tests updated to pass Index accounts to CreateMulticastGroup
• New index_test.rs with tests for Index creation, duplicate rejection, deletion, and rename
• Multicast group tests updated for Index lifecycle during create/update/delete/close
• Onchain allocation tests cover Index + multicast group interaction

[martinsander00]

Superseded — using #3256 directly as PR1 instead