PcbDocReader: length-frame shape-based records; fix trailing-NUL misd…

[Andrei-Errapart]

Summary

ParseShapeBasedRegion guessed whether the properties block carries a trailing NUL by
testing the next byte for zero. That misfires when a record has no trailing NUL and its
outline count is a multiple of 256 (low byte 0x00): the count byte is consumed as the
NUL, every following field shifts by one, and a garbage hole count sends the parser off
the end of the storage. An Agilex board (MicroModule) hits this with a 257-vertex mid-layer
region — one record in 9403 — and because BitConverter reports the overrun as a plain
ArgumentException, which the catch filter did not cover, the whole PcbDoc read crashed.

The records are length-framed, so the parser now uses that: the SubRecord-1 length
(read and previously discarded) bounds the parse, and the trailing-NUL choice is
disambiguated by checking which variant makes the outline plus the length-prefixed
holes land exactly on the record end.

Changes

• ParseShapeBasedRegion reads the SubRecord-1 body length (previously skipped) and
  bounds the whole parse with it; a length that overruns the storage throws
  EndOfStreamException instead of walking off the end.
• Trailing-NUL detection is disambiguated via a new LandsOnRecordEnd helper: accept
  the variant whose outline and holes end exactly at the record end, preferring the NUL
  when both would — preserving the old behaviour for every unambiguous record.
• Every record resyncs to its recorded end after parsing; a record that does not parse
  to its exact length produces a per-record Warning diagnostic instead of poisoning
  the rest of the walk.
• The outer catch filter in ReadShapeBased now also covers ArgumentException and
  EndOfStreamException, so a truly corrupt storage degrades to a warning diagnostic
  rather than crashing the read.

Test Plan

• Existing tests pass (dotnet test) — 850 passed, 0 failed, 10 skipped (tests requiring local sample files)
• New tests added for new behavior
• Manually verified on MicroModule.PcbDoc: full read (1509 components, 9403 regions, no
  diagnostics) and byte-identical round-trip of both shape-based storages.

Checklist

• Code follows existing style and conventions
• Public API changes are documented with XML doc comments (no public API changes —
  all edits are to private parsing methods in PcbDocReader)
• No breaking changes