ltc(v36): advertise raw head at handshake + re-advertise on best-change (root-2)

[frstrtr]

Root-2: genesis-null head advertisement

(a) Bug fixed — genesis-null-advert

On a fresh (--genesis) LTC node the verified share set is empty at the moment of the version handshake. best_share_hash() is verified-only and deliberately collapses to uint256::ZERO once peers exist (so work/template never builds on a MAX_TARGET head). As a side effect the node advertised ZERO in send_version(), so a connecting peer never learned we had a chain and never issued download_shares() for our shares — the chain failed to propagate from a genesis node.

Fix: introduce a separate advertised_best_share() used only on the wire (handshake + re-advertisement). It returns our tallest raw chain head (think's current best, else the tallest m_chain head, else ZERO at true genesis). Work/template selection is untouched and still rides verified-only best_share_hash(). The two concerns are now cleanly split and documented at both declarations.

Re-advertisement: a peer that handshook before we had a chain saw a ZERO advert. readvertise_best() re-announces the head whenever think/clean updates the best share, so that peer pulls our shares on the next cycle.

(b) Livelock-safety — confirmed, no new lock

This change acquires no mutex. advertised_best_share() and readvertise_best() are pure reads over m_best_share_hash/m_chain, invoked from inside run_think() and clean_tracker() which already hold their execution context. No new exclusive lock, no new try_lock, nothing on the compute-thread hot path that could reintroduce the phase2 livelock signature. Net diff is +65/-2 across 2 files.

(c) Deviation from explicit timer — flag for operator

The spec called for an explicit re-advert timer (~10s). This PR instead re-advertises on think-cycle cadence: run_think() after a work refresh, and clean_tracker() when the best share changes. Rationale: it reuses existing event points (no new timer/thread, keeps the no-new-lock guarantee) and re-announces exactly when our head actually moves. If the operator prefers a fixed 10s timer (e.g. to cover a peer that connects during a long quiescent gap with no head change), say so and I will add it as a follow-up.

Scope: LTC only. BTC mirror is tracked separately by btc-heap-opt. Commit b56bfb2 is GPG-signed.

[frstrtr]

Superseded by #104 (e01c160 on ltc-doge/root2-advert-readvert) — the complete Root-2 fix incl. the explicit 10s re-advert timer. This branch (b56bfb2) carries only the on-think/best-change re-advert (half-fix that regresses under a wedged think()). Hold/close pending integrator call on the gated+deploy artifact.

[frstrtr]

Closing per integrator coordination decision: #103 (b56bfb2) is the half-fix. It regresses on the run_think livelock wedge and is superseded by #104 (e01c160), which folds in the 10s core::Timer (removing the deviation that needed an operator opt-in) and the genesis-null-advert raw-head fix. Gate and deploy proceed on #104. Keep closed — do not reopen.