fix: retain graph guidance until the destination layer

[ShiboSoftwareDev]

Problem

The directed graph heuristic stopped when a cross-layer route first left its start layer. For a z0-to-z5 route, z1–z4 are still intermediate layers, so reverting to XY there sends the search into wrong-layer dead ends.

Fix

Keep directed graph guidance on every non-destination layer. Return to the existing XY heuristic only after the candidate reaches the destination layer.

This is a two-line condition change. It does not change topology, layer masks, via legality, or collision rules.

Snapshot

Stacked on #147 and uses its fixture unchanged. The top frame is the same six-layer topology. The bottom frame now shows the completed path: it ignores the z1–z4 exits, leaves the ordinary multilayer region on z5, and reaches the goal.

Hosted verification

• full test and snapshot suite: passed
• srj24 sample 4 benchmark: pending in the autorouter integration PR

All execution and SVG generation ran in GitHub Actions. Nothing was run locally.

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Project	Deployment	Actions	Updated (UTC)
tiny-hypergraph	Ready	Preview	Aug 2, 2026 9:20pm

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