fix(rpc): make verificationprogress tip-aware

[Micah-Shallom]

Summary

Replaces validated / height (which sticks at 1.0 when the chain store stalls) with a time-based formula using wall clock and block timestamps as external references the daemon cannot fudge.

When validated freezes for any reason, now keeps advancing, the denominator grows, and progress visibly falls instead of falsely reporting fully synced.

Why time-based instead of tx-weighted

Bitcoin Core's formula uses per-block cumulative tx counts and a baked-in ChainTxData checkpoint. Both require a chainstore schema migration, which is out of scope for an RPC bugfix. Time-based progress uses two external references Floresta already has (wall clock + block timestamps) and resolves the bug as filed.

Formula

elapsed_validated = validated_block_time - genesis_time
elapsed_total     = max(now, header_tip_time) - genesis_time

verificationprogress = clamp(elapsed_validated / elapsed_total, 0.0, 1.0)

Two external references (now from the OS clock, validated_block_time from the block header) anchor the ratio outside daemon state, so a stalled chain store can't push the value to 1.0.

Test plan

• cargo test -p floresta-node --lib
• CI functional tests

stacked on #948
Partial fix for #1015.