feat(ats): clarify order readiness and enforce model proposal boundary

[AetherAI3]

What changed

• Bump /ats doctor to aether.ats.doctor/2: report local paper activation prerequisites, ATS simulated paper order readiness, provider sandbox and live capital separately. /ats status now labels unavailable order authority and the live HOLD. Existing /1 consumers require explicit migration.
• Introduce aether.ats.model-order-proposal/1, a closed model-facing shape with no account, environment, grant, client, request ID or approval field. Correct the old comment that incorrectly described the private, fully bound equity-order-intent/1 as model input.
• Add shared proposal vectors and independent ATSv2 Python conformance checks in companion ATSv2 #443, whose exact head 4da992be9cca67be47830d268beabd38c6661cd5 passes all workflows, including the paired Agent/ATS fixture gate.
• Add a strict bounded POSIX ustar extractor that parses/validates all entries before writing and rejects link, traversal, special-file, duplicate, Windows special-name and size attacks. The installer accepts custom limits/cancellation. Extraction is not selected by default: there is no published signed archive-format contract or runtime signing anchor yet.
• Add a foundation checkpoint with the remaining gates against the source spec in #162.

Verification

• Local targeted Agent proposal/extractor/doctor tests pass.
• Python canonical fixture checks: 18/18; ATSv2 companion standalone fixture checks pass.
• Local full Agent suite: 2,718 passed, 0 failed (2,722 total with 4 skipped). npm run verify:production passed with 882 packed files and 4,789,137 unpacked bytes. Exact-head CI and CodeQL passed at ef13252d29d229d3c2845c25f83fd2cfaac2d16e (Ubuntu/Windows tests and clean install, supply chain, PyPI launcher).
• Remote tree 10bcde738253726a2fe9c80033b3ec7d1bf7e7f7 matches the reviewed local tree byte-for-byte.

Release boundary

No ATS runtime installer is enabled, no model trading tool is registered, and no simulated paper, sandbox or live order is submitted. G1 requires an independently pinned signing anchor, archive format, entitlement endpoint, launcher and authenticated probe. E1 host, executable data rights, ATS bridge translation, grant/target enrollment, immutable review, separate human approval and reconciliation follow in dependency order. HOLD / NO ORDER SUBMISSION / NO LIVE CAPITAL.

Do not merge #162 or this PR as proof of paper execution.