feat: add reconnectable Gloss executor service

[SunChJ]

Why

Move the long-lived PDF execution boundary into the Gloss-maintained BabelDOC runtime so the macOS app can reconnect, inspect status, and cancel the intended task without depending on Python internals.

What

• add an authenticated loopback service on an ephemeral port with a private ready handshake and runtime identity
• retain reconnectable execution snapshots and sequenced NDJSON events with explicit cursor/replay handling
• make task creation idempotent, enforce one active PDF worker, and add targeted cancellation plus controlled drain/shutdown
• isolate workers in process groups, escalate cancellation with bounded cleanup, and clean descendants after parent loss
• validate request limits, requested output modes, and generated PDF/glossary files inside each task output directory
• canonicalize request paths behind a separator-bounded workroot guard, including traversal, prefix-confusion, symlink, and NUL rejection
• replace request-derived watermark subprocess commands with a fixed, allowlisted spawned-process dispatcher
• document the v1 contract and release the downstream runtime as 0.6.4+gloss.2
• extend clean-wheel Linux/macOS CI to import the real adapter and start/probe/stop the packaged service

Scope

• Gloss still owns the multi-file queue; this service deliberately admits one PDF execution at a time.
• The existing babeldoc CLI remains available during client migration.
• This PR does not include font caching, layout-IR caching, or PDF-save performance work.

Validation

• uv run pytest -q — 110 passed
• uv run ruff check .
• Ruff format check for all changed Python files
• git diff --check
• uv lock --check
• production runner smoke: forkserver warmup, authenticated health/runtime, controlled shutdown
• isolated clean-wheel smoke: adapter imported from site-packages, fake service ready/health/shutdown
• GitHub CI: quality/PDF smoke, Linux/macOS wheel smoke, CodeQL Python/actions analysis, and CodeQL code scanning all passed