refactor: remove non-direct message routing

[LePremierHomme]

Closes #1522

This is a simplified version of the task, where all non-direct message routing support is removed, including sibling-to-sibling.

Rationale

Removes untested complexity before refactoring the IPC contracts codebase. Multi-hop routing was complex, untested, and not currently used in production.

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Note

Refactors cross-net messaging to support only direct parent–child hops; removes multi-hop routing and the postbox/propagation flow.

• Deletes postbox storage, events, getters, and all propagate* code; removes IGateway.propagateAll and related facet methods/selectors
• Enforces direct-only routing: LibGateway.checkCrossMessage rejects non-direct paths with new InvalidXnetMessageReason.NonDirect; applyMsg reverts on non-local destinations
• Simplifies validation by dropping LCA/supply-compat checks; removes LCA inversion in commitValidatedCrossMessage
• Adds NonDirect error enum; cleans up unused errors and methods
• Router/getter facets and checkpointing no longer trigger propagation side-effects
• Tests updated to reflect direct-only behavior (expect reverts for L1↔L3 and sibling↔sibling), adjust token path usage, fix assumptions, and skip multi-hop scenarios
• Selector tables updated to match removed functions

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Direct message validation allows cross-branch messages

Medium Severity

The isDirect validation in checkCrossMessage only checks that route lengths differ by exactly 1, but doesn't verify the subnets are actually in a parent-child relationship. A message from /root/A/B to /root/X (different branch, not the actual parent /root/A) would pass validation because both have route length difference of 1. For BottomUp messages, this would allow the message to be committed to the checkpoint. When the parent /root/A processes it, applyMsg sees destination /root/X doesn't equal current network and triggers a hard revert("Only direct messaging supported"), potentially failing the entire message batch execution.