fix: base validate_unsigned DUST validity window on wall-clock time

[ozgb]

Overview

pallet_midnight's validate_unsigned (the tx-pool validation path) computed the DUST
validity-window reference time tblock from the last produced block's on-chain timestamp,
plus a MaxSkippedSlots margin. When block production stalls (network hiccup, missed AURA
slots beyond MaxSkippedSlots), that stored timestamp goes stale, so tblock lags real time.
A transaction whose creation time (ctime) is near "now" is then wrongly rejected during pool
validation with OutOfDustValidityWindow (ctime > tblock) — even though the next block,
whenever produced, would carry a timestamp ≈ now and accept it.

This change bases tblock on the true expected next block time: the current wall-clock
time rounded up to the next AURA slot boundary, so pool validation tracks real time
regardless of how long production has stalled. The MaxSkippedSlots margin is dropped.

How

• New wall_clock::now_millis() host function (ledger/src/host_api/clock.rs) reading the
  node's system clock. A host function is required because the runtime's validate_transaction
  path registers no offchain extension, so sp_io::offchain::timestamp() would panic there.
• Registered in both the node's HostFunctions executor tuples (node/src/service.rs).
• Injected into the pallet via a new Config associated type WallClockMillis: Get<u64>
  (production wires it to the host fn; mocks return the deterministic on-chain mock timestamp).
• validate_unsigned now sets tblock = wall-clock now rounded up to the next slot boundary.

Determinism is preserved: the wall-clock bump lives only in validate_unsigned (pool
path). Block import/execution goes through the pallet's own pre_dispatch, which uses the raw,
unbumped on-chain timestamp — deterministic across nodes. A non-deterministic wall-clock in
validate_unsigned does not affect consensus.

Removes the now-unused MaxSkippedSlots storage item (it only ever held its default; no setter
exists). This is a metadata change.

Two explanatory TODO comments were added deliberately, noting that once a custom transaction
pool exists, this host fn should be replaced by a timestamp (offchain) extension scoped to the
validation call.

🗹 TODO before merging

• Rebuild runtime metadata (/bot rebuild-metadata) — MaxSkippedSlots storage removed
• End-to-end check: submit a send_mn_transaction on a dev node, confirm no OutOfDustValidityWindow
• Add PR link to the two change files (follow-up commit)
• Ready

📌 Submission Checklist

• All commits are signed off (git commit -s) for the DCO
• Changes are backward-compatible (or flagged if breaking) — see Fork Strategy (node must upgrade with/before runtime)
• Pull request description explains why the change is needed
• Self-reviewed the diff
• I have included a change file, or skipped for this reason:
• If the changes introduce a new feature, I have bumped the node minor version
• Update documentation (if relevant)
• Updated AGENTS.md if build commands, architecture, or workflows changed
• No new todos introduced

🧪 Testing Evidence

• cargo check — 0 errors across midnight-node-ledger, pallet-midnight,
  pallet-cnight-observation-mock, midnight-node-runtime, midnight-node (and
  midnight-node --features runtime-benchmarks, covering the benchmark HostFunctions tuple).
• cargo fmt --check — clean on all modified crates.
• cargo test -p pallet-midnight — 21 passed, 2 ignored. test_validation_works,
  test_validation_fails, and test_pre_dispatch_* all pass (no fixture adjustment needed;
  the new tblock lands closer to ctime than the old +12s, staying inside the window).
• cargo test -p pallet-cnight-observation — 43 passed, 1 ignored (exercises the modified mock).

Please describe any additional testing aside from CI:

• Additional tests are provided (if possible)

🔱 Fork Strategy

• Node Runtime Update
• Node Client Update
• Other:
• N/A

Adding a host function means a node must have it registered before running a runtime that calls
it. An old node importing/validating with a new runtime would fail to resolve the
wall_clock::now_millis wasm import — upgrade nodes with (or before) the runtime.

Links

Closes #1856

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[jina-simulation]

Jina Review

Jina was blocked while reviewing this PR.

Review: https://app.usejina.com/reviews/a52aebb1-ffbb-471f-8cbb-17032fa16ef2

Item	Status
Review	Blocked
Findings	Insufficient credits

[ozgb]

/bot rebuild-metadata

[ozgb]

/bot rebuild-metadata