chore: remove helium network mode

[simone-stacks]

Description

Helium used a legacy Stacks 2.x run loop (helium::RunLoop) incompatible with Epoch 3 / Nakamoto. No signers, no block proposals, no tenure changes. This PR removes it entirely, including the Node/Tenure structs and helium-only integration tests.
Shared genesis utilities move to genesis.rs, which neon and nakamoto still use.

Applicable issues

• fixes cleanup: remove deprecated helium logic #6408

Additional info (benefits, drawbacks, caveats)

Stacked on #7155 (mocknet removal). The diff narrows to the helium-only changes once that one merges.

Checklist

• Test coverage for new or modified code paths
• For new Clarity features or consensus changes, add property tests (see docs/property-testing.md)
• Changelog fragment(s) or "no changelog" label added (see changelog.d/README.md)
• Required documentation changes (e.g., rpc/openapi.yaml for RPC endpoints, event-dispatcher.md for new events)
• New clarity functions have corresponding PR in clarity-benchmarking repo

[coveralls]

Coverage Report for CI Build 27563596064

Coverage increased (+0.07%) to 85.782%

Details

• Coverage increased (+0.07%) from the base build.
• Patch coverage: 4 uncovered changes across 3 files (82 of 86 lines covered, 95.35%).
• 8332 coverage regressions across 138 files.

Uncovered Changes

File	Changed	Covered	%
stacks-node/src/genesis.rs	73	71	97.26%
stacks-node/src/main.rs	1	0	0.0%
stacks-signer/src/config.rs	4	3	75.0%
Total (5 files)	86	82	95.35%

Coverage Regressions

8332 previously-covered lines in 138 files lost coverage.

Top 10 Files by Coverage Loss	Lines Losing Coverage	Coverage
stackslib/src/chainstate/burn/db/sortdb.rs	433	90.03%
stackslib/src/chainstate/nakamoto/mod.rs	403	84.62%
stackslib/src/config/mod.rs	362	71.62%
stackslib/src/net/mod.rs	317	77.91%
stackslib/src/chainstate/stacks/index/storage.rs	277	82.41%
clarity/src/vm/database/clarity_db.rs	269	82.06%
stackslib/src/chainstate/stacks/miner.rs	264	82.79%
stackslib/src/chainstate/stacks/db/transactions.rs	242	97.11%
stackslib/src/net/inv/epoch2x.rs	213	79.95%
stackslib/src/net/chat.rs	200	93.03%

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Coverage Stats

Relevant Lines:	223841
Covered Lines:	192016
Line Coverage:	85.78%
Coverage Strength:	19096614.84 hits per line

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💛 - Coveralls