feat(common-line): place parts on common lines automatically at a chosen level

Slice 3 of common-line cutting. On costly plate (N26-295: 3/16 RA310S at about
$2,500 a sheet) pairing parts one kerf apart fits the part the spacing cannot:
the last plate's 38 parts fit on one 48x120 with Auto Nest only with pairs.

- Drawing Properties gains "Allow common line" (Drawing.AllowCommonLine, off by
  default because customer edge-quality needs vary). The nest's CommonLineUsage
  (Off / When it adds parts / Whenever available) is set in Auto Nest and seeded
  from Tools > Nest Defaults; both persist (nest.json, defaults.json).
- NestPipeline runs the ordinary solve, then, with a level on, trials where
  eligible parts are offered as kerf-gap pair compounds. Placed pairs expand into
  parts plus explicit NestJobCommonLine groups, are validated again, and
  CommonLineRanking keeps a trial only by the level: count first, then sheets;
  "when it adds parts" keeps the plain layout on a tie, "whenever" also takes a
  shorter cut or less material, never at the cost of a part. Engines, FillScore
  and their comparers are unchanged; Off asks the engine exactly what it did.
- NestLayoutCheck exempts members only through a sheet's explicit group list and
  only when the job allows common lines; materializing and committing create
  Plate.CommonLines.
- Server nesting writes common-line jobs as version 3 and replays the recorded
  trial answers, so the desktop binds the same groups.
- Fill Area honors the level through a PlateFillService overload.
- The benchmark gains --common-lines <level> --kerf <k>.

Benchmark (synthetic nests + N26-295, --parallel 1): with no flag and with
--common-lines off every engine matches the base commit; with a level on no
layout becomes invalid and three runs save a sheet at equal nested area.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/automatic-nesting.md

@@ -6,6 +6,7 @@
 2. Resolve the selected name through `NestingEngineRegistry` (built-ins and plug-ins use the same path).
 3. Bundle parts that fit other parts' enclosed cutouts into their frames ([filling cutouts](nesting-engines.md#filling-cutouts)); a refused or less complete bundled trial is replaced by the ordinary solve.
 4. Run the engine without mutating caller drawings or plates.
+   When the job's common-line level is on, solve once more with eligible parts offered as kerf-gap pairs and keep that layout only by the level's ranking ([automatic common-line placement](common-line-cutting.md#automatic-placement)).
 5. Check returned placements with the independent `NestLayoutCheck` used by the benchmark (for bundles, again after expanding them to physical frames and inserts).
 6. Consolidate exact ordered nonempty layouts only when the entire result is valid, then bind each layout to the caller's drawing references; the caller decides whether to commit it.