docs: repair CHANGELOG structure and add the v0.7.0 migration guide

[s2x]

Follow-up to the v0.7.0 milestone merge (#229–#235). No code change; the suite runs green before and after.

What was broken

Merging seven branches into a moving main left the [Unreleased] section structurally broken:

• Four consecutive ### Fixed headings. Keep a Changelog expects one per category, and GitHub renders the duplicates as separate lists, so entries after the first looked detached from the section.
• Four missing blank lines between adjacent entries, which merged them into a single Markdown list item — the entries after the first were swallowed visually.

Only [Unreleased] is touched. The historical sections keep their formatting, including their own pre-existing duplicate headings, which are not this PR's business.

What was missing

The test-command change was not in the changelog at all. That is the one thing in this milestone that can break someone's workflow, and it was undocumented. .github/scripts/run-tests.sh already handled both halves correctly, so the defect was documentation only.

The open allocator finding (#228) was invisible. It is a real undefined-behaviour issue with the prebuilt SDK, currently benign but not going away. Added under a new ### Known issues heading, explicitly marked as not the cause of the exit crash that was fixed, because that confusion already happened twice during this milestone.

MIGRATION.md had no v0.7.0 section

Six additive API changes, a prebuilt SDK switch, and the invocation change — with nothing written down. Added a full section, plus a file-level index at the top since it now covers two upgrades. Each entry states whether it is backward compatible; only the test command is not, and that is called out on its own.

Guarding against a repeat

test_docs_consistency.phpt now asserts that the migration guide exists, states the change is additive, and documents each of the eight new public API names:

v0.7.0 section: present
v0.7.0 states additive: yes
forIvfRabitq               documented
setIvfRabitqParams         documented
twoPassBuild               documented
setVamanaPrefetch          documented
jiebaDictDir               documented
ftsBruteForceByKeysRatio   documented
getIoBackendType           documented
indexParams                documented

Without that check the guide silently falls behind the README again — which is exactly how it ended up with no v0.7.0 section.

Verification

Number of tests : 202               202
Tests skipped   :   0 (  0.0%)
Tests failed    :   0 (  0.0%)
Tests passed    : 200 ( 99.0%)

test_dbs/ is left with only .gitignore.

Note

This is deliberately not a 0.7.0 release. The milestone still has open issues (#222, #226, #217, #219, …), and the final ## [Unreleased] → ## [0.7.0] cut plus a tag is a separate step once those land. Doing it now would just need redoing.

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