docs: mark port 5564 as assigned to slotstream in PORTS reserved ranges (#6031)

[atomantic]

Summary

Fixes the 5562-5569 reserved-ranges summary in docs/PORTS.md (line 135), which incorrectly listed port 5564 as unassigned. Port 5564 is allocated to portos-slotstream (SLOTSTREAM: 5564 in ecosystem.config.cjs and server/lib/ports.js, plus the port table and on-demand notes in the same document — overlooked when #5815 added slotstream). The summary now lists 5564 slotstream (on demand) as assigned and narrows the unassigned range to 5565-5567, 5569.

Test plan

• Verified port 5564 against the canonical sources before editing: SLOTSTREAM: 5564 in both ecosystem.config.cjs and server/lib/ports.js.
• npm test -- lib/ports.test.js (server workspace): 7 passed.
• Self-reviewed the diff: one line in docs/PORTS.md, no other files touched.
• Local reviewer (mtplx, low effort) returned no verdict — no model is configured for the mtplx reviewer in this environment. Recorded as review-blocked per swarm protocol.

Closes #6031

[atomantic]

Review-blocked: the mtplx local reviewer returned no verdict (no model configured for the mtplx reviewer in this environment). The change is a docs-only one-liner verified against the canonical PORTS sources, self-reviewed, with ports tests passing — leaving the PR open for CI/human review.