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>

package.json

@@ -8,7 +8,7 @@
     "road-compiler": "bin/road-compiler.js"
   },
   "scripts": {
-    "test": "node test/index.js && node test/native-road-edits.js && node test/road-revisions.js && node test/fixtures.js",
+    "test": "node test/index.js && node test/native-road-edits.js && node test/road-revisions.js && node test/direct-edit-solver.js && node test/fixtures.js",
     "test:fixtures:update-baseline": "node test/update-fixture-baselines.js",
     "road:workbench": "node bin/road-workbench.js",
     "road:export": "node bin/road-compiler.js",