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/verify.cts

@@ -22,7 +22,7 @@ import stateMod = require('./state.cjs');
 import modelProfilesMod = require('./model-profiles.cjs');
 // eslint-disable-next-line @typescript-eslint/no-require-imports -- plan-scan.cjs is an export= CommonJS module
 import planScanMod = require('./plan-scan.cjs');
-import { execGit, platformReadSync as safeReadFile, platformWriteSync } from './shell-command-projection.cjs';
+import { execGit, platformReadSync as safeReadFile, platformWriteSync, posixNormalize } from './shell-command-projection.cjs';
 import { PACKAGE_NAME } from './package-identity.cjs';
 import { formatGsdSlash, resolveRuntime } from './runtime-slash.cjs';
 import { detectSchemaFiles, checkSchemaDrift } from './schema-detect.cjs';
@@ -1157,7 +1157,7 @@ function cmdValidateConsistency(cwd: string, raw: boolean): void {
 
       for (const dir of dirs) {
         const phasePath = path.join(phaseRoot, dir);
-        const phaseLabel = path.relative(planBase, phasePath).replace(/\\/g, '/');
+        const phaseLabel = posixNormalize(path.relative(planBase, phasePath));
         const phaseFiles = fs.readdirSync(phasePath);
         const plans = phaseFiles.filter((f) => f.endsWith('-PLAN.md')).sort();