Align crossversion refusal assertions with the 0.9.0-dev naming

[aaltshuler]

What & why

Main's Test Workspace is red (run 30180551717) at crossversion_upgrade.rs:891: the genuine-v8 cell asserts the v8 refusal contains 0.12.x, but the 0.9.0 release bump corrected the schema-to-release map — stamps v5–v8 never shipped in a published release, so their refusals now name 0.9.0-dev instead of fictional release lines.

Four cells asserted the old strings (v5/v6/v7/v8). Only the v8 cell runs in CI (it builds a genuine predecessor binary from 725793af; the others skip without their env seams), which is why every local gate stayed green while post-merge CI went red.

Backing issue / RFC

• Trivial fast-lane — test-string alignment with an already-merged user-facing message correction; no behavior change.

Checklist

• Change is focused (four assertions + two testing.md sentences)
• Tests updated — this is the test fix
• Public docs: N/A (upgrade.md and the release notes already document the 0.9.0-dev naming)
• Reviewed against docs/dev/invariants.md — no invariant touched

Validation

CI cannot prove this pre-merge (Test Workspace is post-merge-only), so it was validated against the real boundary locally: built the genuine v8 binary from 725793af (reports omnigraph 0.8.1 / internal-schema 8) and ran the exact failing cell with OMNIGRAPH_V8_BIN set:

test current_v9_refuses_and_rebuilds_genuine_v8_and_v8_refuses_v9 ... ok

The v5/v6/v7 cells received the identical fix; they are env-gated and skip in CI by design.

Greptile Summary

Aligns storage-version guidance with the canonical schema-to-release mapping.

• Updates genuine v5–v8 cross-version refusal assertions to expect 0.9.0-dev.
• Corrects developer, RFC, and operator documentation to state that schemas v5–v8 were unpublished development formats and schema v9 maps to the 0.9.x release line.
• Resolves the previously reported inconsistency between testing guidance, versioning policy, and the upgrade guide.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains; the revised assertions and documentation consistently match the implemented schema-to-release mapping, and the previous guidance inconsistency is resolved.

Important Files Changed

Filename	Overview
crates/omnigraph-cli/tests/crossversion_upgrade.rs	Updates all genuine v5–v8 refusal assertions to match the implementation’s canonical 0.9.0-dev label.
docs/dev/testing.md	Corrects cross-version test guidance for the unpublished v5–v8 formats.
docs/dev/versioning.md	Aligns the canonical storage-version policy with v5–v8 as development formats and v9 as the 0.9.x release format.
docs/dev/writes.md	Corrects the documented release associations for internal schema versions v7 and v8.
docs/rfcs/0023-key-conflict-fencing.md	Updates RFC history to identify schema v6 as an unpublished 0.9.0 development format.
docs/user/operations/upgrade.md	Aligns operator-facing rebuild guidance and version tables with the implemented schema mapping.

_{Reviews (2): Last reviewed commit: "docs: retire the fictional 0.10.x–0.13.x..." | Re-trigger Greptile}