v0.5.1 — adds DSH 0.2.0-rc.2 to the tested host matrix

What changed

• Runtime version guard, installer gate, CLI guidance and help text now accept exactly 0.1.7-alpha.1 or 0.2.0-rc.2 (homogeneous host-package sets), all derived from a single constant (TARGET_DSH_VERSIONS) so they cannot drift.
• DEFAULT_SOURCE pinned to v0.5.1; installer tests assert the source pin follows the package version, cover the rc.2 launcher, and refuse adjacent unvalidated versions (0.1.7-rc.1, 0.2.0-rc.1, 0.2.0).

Verification (tag v0.5.1, commit 3b45e7a)

• Full repo check suite (unit, legacy-A, compaction A/B) passes; 168/168 + check green.
• Live on isolated host 0.2.0-rc.2: install via official dsh plugin add, boot with zero activation failures, Codex Native preset registered, real ChatGPT-account /compact committed a native checkpoint (responses.compaction-trigger.v2 envelope, no error), UI reported "10 history records compacted".

Known issue (0.2.0-rc.2)

After a host restart, continuing a conversation that contains a committed native checkpoint fails during request preparation (CODEX_RUNTIME_REQUEST_PREPARE); fresh sessions and the compaction itself work. Behavior on 0.1.7-alpha.1 for the same flow is unknown (not reproduced there; a same-version control experiment could not rebuild the OAuth session cheaply). Until fixed, avoid relying on native-checkpoint resume across restarts: keep long Codex Native sessions within one host run or use reader-text.