fix(#511): allow absolute completion generations after I5 on resume

[James3014]

Refs #511 and Nexus-new #1384. The Nexus completion loop budgets at most five new generations per invocation, relative to its starting generation. The connector-host parseCompletionGeneration incorrectly also caps the absolute generation at five: a resumed run starting at I3 reaches I6 within its allowed five generations, but the host rejects generation_INVALID before the intended checks.

Minimal fix: validate the absolute generation as a positive safe integer while keeping the user-facing maxGenerations <= 5 budget unchanged. No approval, merge/CAS, verifier, workflow, permissions or CI matrix change. Existing unrelated #343 handles different G07 methods and is not modified.

RED first: test-only commit 1d8c4e036090ced325c1bc7539cc231e2f81ef6d is intentional and should fail the macOS exact-head CI test on I6. After observing RED, add the one-line production fix and verify the final exact head, including the unchanged maxGenerations=5 checks. Nexus-new #1384 AC11 live multi-merge proof and Gateway activation remain separate gates.

[James3014]

Independent read-only review on exact head 4c4993014d0fc4dd0cd2d6817edd9f987e315192 / base 974e8876f7c8dc9818f99f67c8a29a8bd06443f9: DevSpace M4 durable agent agt_8880d6d2 (agy-medium-review, configured Gemini 3.8 Flash Medium; provider model attestation metadata_only) returned terminal RECOMMEND_MERGE, with no changed paths. Review confirmed only two scoped files, parseCompletionGeneration now accepts positive safe absolute generation numbers while maxGenerations remains capped at five per call; no privilege/merge gate changes or semantic overlap with old #343. Independent reviewer result is advisory and is not a Nexus v4 Candidate acceptance receipt.

RED macOS CI: run 38492574701 on test-only 1d8c4e0 terminal failure at generation_INVALID I6. GREEN macOS CI: run 38492817361 on final head terminal success (typecheck/test/build/doctor). Nexus Core issue run and completion are still failing on this head because the nexus/connector-host-v1 line lacks .nexus-core/config.toml; they are not declared green. Fresh GitHub branch rule query reports no required status rules on this specialized branch. This note grants no deployment or #1384 AC11 live pilot certification.