refactor(#2246): centralize cross-platform path-separator handling (toPosixPath / toNativePath / posixNormalize) (#2247)

Replace every open-coded separator translation across the installer/hooks
source with named, tested seams in shell-command-projection.cts (the platform
seam), removing all hardcoded `/`+`\` from path handling:

- toPosixPath(p)   — this machine's native path → POSIX (running-OS relative;
                     for local filesystem paths).
- toNativePath(p)  — POSIX → native (collapses the win32 `/\//g,'\\'` ternary).
- posixNormalize(p)— unconditional `\`→`/`, OS-independent; for emitting paths
                     to a POSIX/bash TARGET (which may differ from the running
                     OS) and for parsing mixed-separator input.

core-utils.toPosixPath now delegates to the seam, so its 20+ existing consumers
resolve to one implementation; no duplicate helper.

- ~47 sites across runtime-hooks-surface, runtime-artifact-conversion,
  runtime-artifact-install-plan, drift, init, worktree-safety,
  installer-migrations, installer-migration-authoring, install-engine, surface,
  verify, runtime-artifact-layout, schema-detect, check-command-router.
- Closes the latent POSIX-literal-backslash corruption class (the regex form
  corrupts a POSIX path containing a literal backslash; split(path.sep) does not).
- New unit + fast-check property tests for all three helpers.

Closes #2246

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/install-engine.cts

@@ -27,6 +27,7 @@ import runtimeArtifactLayout = require('./runtime-artifact-layout.cjs');
 import runtimeArtifactInstallPlan = require('./runtime-artifact-install-plan.cjs');
 import runtimeNamePolicy = require('./runtime-name-policy.cjs');
 import installProfiles = require('./install-profiles.cjs');
+import { posixNormalize } from './shell-command-projection.cjs';
 
 const { processAttribution } = runtimeArtifactConversion;
 // resolveRuntimeArtifactLayout: accessed via module ref (not destructured) so
@@ -970,8 +971,8 @@ function installOpencodeFamilyArtifacts(
     isGlobal,
     isOpencode: behaviors.skipHomePrefixSubstitution === true,
     isWindowsHost: process.platform === 'win32',
-    resolvedTarget: path.resolve(configDir).replace(/\\/g, '/'),
-    homeDir: os.homedir().replace(/\\/g, '/'),
+    resolvedTarget: posixNormalize(path.resolve(configDir)),
+    homeDir: posixNormalize(os.homedir()),
   });
 
   const commandDir = runtimeArtifactInstallPlan.assertDestWithinConfigHome(configDir, 'command');