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/schema-detect.cts

@@ -8,6 +8,8 @@
  * This module does not read the filesystem directly.
  */
 
+import { posixNormalize } from './shell-command-projection.cjs';
+
 // ─── ORM Patterns ───────────────────────────────────────────────────────────
 
 export interface SchemaPattern {
@@ -82,7 +84,7 @@ export function detectSchemaFiles(files: string[]): DetectSchemaFilesResult {
   const matches: string[] = [];
   const orms = new Set<string>();
   for (const rawFile of files) {
-    const file = rawFile.replace(/\\/g, '/');
+    const file = posixNormalize(rawFile);
     for (const { pattern, orm } of SCHEMA_PATTERNS) {
       if (pattern.test(file)) {
         matches.push(rawFile);