Bug: a declared plane cannot be stitched on a fine-pitch BGA, and the suggested remedy does not exist #165

What were you trying to achieve?

Export a four-layer board whose two rails are planes, with a 121-ball 0.8 mm BGA on them.

What are the expected results?

The board declares requires via_in_pad and a tier that fills, so the stitching via goes in the pad.

What are the received results?

Export fails. plane_via_offset only ever places beside the pad, and nothing can grant via-in-pad here.

synth: kicad export failed: power-plane fanout failed: net `net_8`: no legal
stitching-via position beside the pad at (51.85, 39.96) mm on Top: every
candidate is outside the board or within clearance of another pad or via.
Move the part, widen the board, or allow via-in-pad explicitly.

The board declares requires via_in_pad and manufacturer "jlc-filled-via",
and synth validate exits 0 on it, so ERC agrees the process is available.
The export still refuses.

This is not a placement accident. The failing pad is on a plane net, and the
only component within reach of it is the BGA. Its 21 rail-ball connections sit
in an 11 x 11 grid at 0.8 mm pitch, so an interior rail ball has neighbours on
all four sides 0.8 mm away. A beside-pad via needs 0.582 mm of clearance from
pad copper at this floor — 0.3 mm drill plus 2 x 0.13 mm ring is 0.56 mm across
— and 0.400 mm is all there is. Beside-pad stitching is arithmetically
impossible for those balls, which is the same arithmetic that makes via-in-pad
mandatory for the escape.

Steps To Reproduce

1. Declare two netclasses with plane, join a rail net to each, and put a
   121-ball 0.8 mm BGA's supply balls on them.
2. Declare requires via_in_pad and manufacturer "jlc-filled-via".
3. synth validate — exits 0.
4. synth export-kicad — fails as above.

In what environment did you encounter the issue?

synth main at 157f6ea plus #155, #159, #160, #161 and #164. KiCad 10.0.6, Linux.

Additional information you deem important

The error names a remedy that does not exist: --krt-allow-via-in-pad reaches the router, never build_pcb.

crates/synth-kicad/src/pcb.rs:581, fn plane_via_offset(pad, origin, outline, obstacles), takes no via-in-pad parameter and has no in-pad branch. The
comment above the plane emission says the via is placed beside the pad "so the
board does not silently depend on via-in-pad" — reasonable when nothing could
declare it, but #155 now lets a board declare exactly that, and the exporter
does not read it.

So the suggestion is unreachable by any route: not by flag, since
krt_allow_via_in_pad is a router option; not by profile, since build_pcb
takes no FabricationPolicy; and not by declaration, since the board's
required_processes never reaches the exporter.

Fixing it looks like threading the granted permission into build_pcb and
letting plane_via_offset return the pad centre when via-in-pad is allowed and
no offset position is legal. Happy to pick this up.