feat: insert direct edit constraint solver stage

Adds resolveDirectEditConstraints between compileRoadModel and
compileGeometry, per the parent design's data flow. The stage is an
identity transform until the individual constraint kinds land: it
returns empty road profiles, junction plans and handle manifest, so
compileGeometry's geometry code is untouched and output stays
byte-identical.

Constraint planning is separated from solving up front. Only `exact`
and `recheck` constraints may reach the solver; `disabled`,
`pending`, `conflicted` and `stale` are reported as unapplied with a
reason. SOLVED_KINDS starts empty and grows one entry per kind, so a
partially delivered solver reports what it skipped instead of
publishing half-solved geometry.

diagnostic() moves to src/compile/diagnostics.js so the solver can
share the compiler's diagnostic shape without depending on fs, which
would break the pure-function boundary that lets preview, the full
compile and the CLI export share one implementation.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.trellis/tasks/08-26-direct-edit-solver-api/task.json

@@ -3,7 +3,7 @@
   "name": "direct-edit-solver-api",
   "title": "约束求解器与编辑 API",
   "description": "resolveDirectEditConstraints 纯函数边界，以及预览/保存/revision/rebase API",
-  "status": "planning",
+  "status": "in_progress",
   "dev_type": null,
   "scope": null,
   "package": null,