fix: #810 — path.posix.* and path.win32.join sub-namespace dispatch

[proggeramlug]

Summary

• path.posix.<method>(...) and path.win32.<method>(...) returned undefined because the sub-namespace shape was excluded from the generic mod.X.Y() arm with no replacement lowering.
• Adds HIR lowering that routes path.posix.* to the existing POSIX Expr::Path* variants (direct shape rewrite — runtime is already POSIX) and routes path.win32.join to a new Expr::PathWin32Join backed by a dedicated js_path_win32_join runtime function that joins with \ and normalizes drive letters / mixed separators.
• test_parity_path.ts is now a full byte-for-byte PASS against node --experimental-strip-types.

Scope notes

• Methods covered for posix: join, dirname, basename, basenameExt, extname, resolve, isAbsolute, relative, normalize, parse, format, toNamespacedPath, matchesGlob.
• Methods covered for win32: join only. Other path.win32.* methods are intentionally left unwired — falling them through to the POSIX runtime would silently mis-handle drive letters and \ segments. Track those in a follow-up if needed.

Test plan

• 4-line repro from the issue prints output matching Node (posix.join: /foo/bar, win32.join: C:\foo\bar).
• test-files/test_parity_path.ts diff vs Node is empty.
• cargo test --release -p perry-hir -p perry-codegen passes.

Closes #810.