feat(daemon): enable private Windows named-pipe service

[jlongster]

Summary

• Enable the shared Windows daemon with protocol 7 and the unchanged exclusive-owner/handoff state machine.
• Store registration and locking state in LOCALAPPDATA/opencode-pty or an absolute OPENCODE_PTY_RUNTIME_DIR override. Enforce current-user protected ACLs, ownership/reparse checks, non-delete-shared directory/lock handles, and instance-scoped cleanup.
• Publish atomically using FileRenameInfoEx replace/POSIX flags on the held private source. The full destination comes from GetFinalPathNameByHandleW on the held directory; RootDirectory is NULL for the supported Win32 wrapper. Open readers retain the old complete file; new readers see the new complete file. No delete gap, source reopen, ACL relaxation, or non-atomic fallback.
• Retain pipe namespace ownership through cleanup; cancel partial requests and blocked subscribers before service shutdown; keep the cleanup watchdog on listener-error paths too.
• Expose only a minimal PipeConnection byte stream with optional I/O timeouts, not Windows TerminalClient/interactive CLI parity.
• Add five real daemon tests and two private/atomic storage regressions. Reuse P2's shared console fixture without divergent setup.

Dependency correction

The original standalone T3 ancestry exposed an ARM64 owner-loss cleanup watchdog failure under blocked subscriber output. This PR was rebased onto the full P2 runtime cleanup series, not a partial fix. The strict failing daemon regression is unchanged and now passes on both native architectures. Runtime, service, and shared-fixture bytes are identical to the frozen P2 base.

Verification — head 040087b

• Windows run 33787060709: native x64 and ARM64 each pass 34 library + 5 callback-effects + 12 real runtime + 1 retained-pointer + 5 real daemon tests = 57 tests, plus the standalone 34-library-test rerun. The two ignored tests are subprocess fixture entrypoints, not skipped coverage. Native Clippy, formatting, architecture checks, and executable smoke tests pass.
• The daemon suite verifies authentication/live-owner exclusion/handoff, lock and stale-instance handling, unclaimed partial-request cancellation, real ConPTY create/input/output/resize/shutdown, and owner-loss cancellation of a blocked subscriber plus partial request with the child process signalled.
• Core run 33787060669: Linux 61 tests, macOS 56 tests, and Miri 5 callback + 1 retained-pointer test under each borrow model pass; formatting and locked Clippy pass.
• Local quick checks: 48 selected tests, locked all-target/all-feature Clippy, formatting, actionlint, both retained-pointer Miri modes, and range-diff inspection pass.

Unmerged stack

master -> windows-transport (#13) -> windows-pipes (#14) -> conpty-runtime (#12) -> conpty-cleanup (#15) -> windows-daemon (this PR).

Base: conpty-cleanup at c92abf2e6587dac215a2f73e0c0861c162507884.

No merges or publication. Source is frozen pending coordinator-led verification. Basic coverage does not claim exhaustive lifecycle coverage, repair the documented Linux blocked-master-write limitation, or add Windows interactive CLI parity.