ltc(seeds): autosense seed ladder (WIP)

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WIP / DRAFT. Additive seed-discovery ladder for LTC (DOGE aux rides it).

Scope: additive-only in src/impl/ltc/coin/chain_seeds.hpp. No src/core touch.

Base/stack correction: base = origin/master @ ddbd81e, which ALREADY contains #911 (merge c1d3cef, merged 2026-07-27 10:42Z) providing f20cb82 is_wire_advertisable. No cherry-pick needed — the merge-order-#911-then-mine plan was against a stale local mirror; verified is_wire_advertisable present on ddbd81e.

This commit (d679a7c): SeedRung enum + SeedLadderRung (settle_ms/min_peers escalation keys) + ltc_seed_ladder() composing existing ltc_dns_seeds()/ltc_fixed_seeds(). Autosense = stay on highest rung yielding >= min_peers within settle_ms, escalate on starvation.

Remaining (WIP): (1) pin the EmbeddedBootstrap rung host from the live-bootstrap CI gate (own seed host, not public mainnet) — currently stubbed empty; (2) wire ltc_seed_ladder() into the node discovery loop consumer; (3) DOGE aux ladder-share confirmation; (4) ladder-ordering unit KAT.

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CI red triage (head d679a7c): the single failing check is Windows x86_64, and it is transient runner infra, not a code defect and not the #932 checkout-jam signature.

• actions/checkout@v6 SUCCEEDED (the ci: git-fetch-retry composite action (HTTP/1.1 + bounded backoff) #932 family is checkout CANCELLED + real step SKIPPED — not the case here).
• actions/setup-python@v6 could not be downloaded: HttpClient.Timeout of 100 seconds elapsing from codeload.github.com, then backed off.
• Actual failing step: Build secp256k1 (exit 1) on the Windows runner.

Linux x86_64, the full coin-matrix (ltc/doge/dash/dgb/bch/btc smoke), and LTC gate G3a are all GREEN. An additive chain_seeds.hpp seed-ladder cannot selectively break Windows secp256k1 compilation. Disposition: re-run the Windows job; no code change owed. Leaving the PR as-is.