feat: add native Windows WorkBundle support

[Ldsystem]

Summary

• replace the Bash installer pair with one Python 3.13 source-tree/source-archive installer
• add cross-platform locking, durable writes, path safety, POSIX symlink and Windows junction behavior
• replace username/password protected-fd transport with one redacted stdin-JSON contract
• run the canonical Python gate on Ubuntu, macOS, and Windows, including a native Windows archive-install exercise

Local evidence

• platform task: 123 related tests passed, 3 native-platform skips
• installer task: 33 focused tests passed, 2 native-Windows skips; 110 related tests passed
• credential task: 31 focused tests passed; 90 related tests passed, 2 native-platform skips
• CI convergence: 26 CI-contract tests passed; 222 focused tests passed, 5 skipped; 43 tests passed from an exact Git archive without .git
• each task received distinct semantic review and bounded repairs; final frozen head review returned accept with no findings

Hosted CI is the remaining platform observation. Its result is evidence for controller assessment, not automatic semantic acceptance.

[Ldsystem]

Superseded by #44, which contains the reconciled WOR-120 implementation, passed the exact Ubuntu/macOS/Windows matrix, and has now been merged. Closing this legacy integration PR without merging.