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/core-utils.cts

@@ -24,12 +24,18 @@ const { comparePhaseNum } = phaseIdModule;
 // eslint-disable-next-line @typescript-eslint/no-require-imports
 import planningWorkspace = require('./planning-workspace.cjs');
 const { findContextMdIn } = planningWorkspace;
+// eslint-disable-next-line @typescript-eslint/no-require-imports
+import shellCommandProjection = require('./shell-command-projection.cjs');
 
 // ─── Path helpers ────────────────────────────────────────────────────────────
 
-/** Normalize a relative path to always use forward slashes (cross-platform). */
+/**
+ * Normalize a relative path to always use forward slashes (cross-platform).
+ * Delegates to the single separator seam in shell-command-projection so there is
+ * exactly one implementation of native→POSIX conversion across the codebase.
+ */
 function toPosixPath(p: string): string {
-  return p.split(path.sep).join('/');
+  return shellCommandProjection.toPosixPath(p);
 }
 
 /**