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/check-command-router.cts

@@ -38,7 +38,7 @@ const { evaluatePredicate } = gatePredicateEval;
 // eslint-disable-next-line @typescript-eslint/no-require-imports
 import apiCoverageMod = require('./api-coverage.cjs');
 const { detectApiIntegration, validateCoverageMatrix } = apiCoverageMod;
-import { execTool } from './shell-command-projection.cjs';
+import { execTool, posixNormalize } from './shell-command-projection.cjs';
 
 // ─── Helpers ──────────────────────────────────────────────────────────────────
 
@@ -1035,7 +1035,7 @@ function cmdApiCoverageVerifyPre(projectDir: string, args: string[], raw: boolea
   // .planning/phases/ (or a milestone archive). The raw arg is never used as a
   // path, so `..`, absolute paths, and arbitrary directories cannot reach a
   // file read. Mirrors cmdVerifySchemaDrift's token-match approach.
-  let token = phaseArg.replace(/\\/g, '/').split('/').filter(Boolean).pop() || '';
+  let token = posixNormalize(phaseArg).split('/').filter(Boolean).pop() || '';
   // A token like ".." or "." carries no phase identity → unresolvable.
   if (token === '.' || token === '..') token = '';