feat(scoring): add public diagnostics for ChannelLiquidities

[ovitrif]

This PR adds a public read-only diagnostic surface to ChannelLiquidities and ProbabilisticScorer so external tools can inspect serialized scorer files without needing access to the private liquidity_history internals.

Depends on:

• feat(tools): add scorer-inspect CLI with text/csv/json output ldk-node#79 — adds a scorer-inspect CLI that consumes these accessors
• feat: add LDK debug action to export pathfinding scores bitkit-android#915 — sibling work that uses the existing exportPathfindingScores() to produce the bytes the inspector reads

Summary

• New pub struct ChannelLiquidityDiagnostic exposing per-SCID liquidity offsets, last-updated timestamps, and the raw 32-bucket weight arrays from HistoricalLiquidityTracker plus its total_valid_points_tracked summary.
• New methods ChannelLiquidities::diagnostics() and ProbabilisticScorer::diagnostics() returning Vec<ChannelLiquidityDiagnostic>, sorted by SCID for deterministic output.
• Two small pub(super) accessors (HistoricalBucketRangeTracker::buckets, HistoricalLiquidityTracker::total_valid_points_tracked) so the diagnostic function can read bucket state without exposing the internal types.
• No behavior change: the existing scoring path is untouched.

Directional resolution (turning the non-directional offsets into per-direction sat values) is intentionally left out of this surface — that requires a NetworkGraph for capacity and node-id ordering and is best done by the consumer.

The motivation is a separate scorer-inspect CLI (linked above) that reads .bin files served at URLs like scores.zeusln.com/latest.bin or produced locally by Node::export_pathfinding_scores, and emits text/CSV/JSON diagnostics. With these accessors that consumer can be a thin shim instead of duplicating internal types.

Test plan

• New unit test diagnostics_distinguishes_history_populated_from_empty builds a scorer with one probe-driven entry and one empty entry, calls diagnostics() on both ChannelLiquidities and ProbabilisticScorer, and verifies the populated/empty distinction is preserved through a Writeable -> Readable round-trip on both wire shapes.
• cargo test -p lightning --lib routing::scoring — all 33 scoring tests pass.

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