AG fixture: state the commit-time conclusion the guidance rests on (#991)

[erikdarlingdata]

Evidence file and CHANGELOG only, no code. Two files.

#1708 corrected the AG lag guidance to "do not derive lag from commit times at all", and cites tools/ag-fixture/VALIDATION.md. The file carried both halves of that argument - the freeze in section 6b, the quiet-database case in the last_commit_time subsection - but never drew them together. A reader who found one half could reasonably conclude the other case was safe to guard against, which is precisely the mistake #1708 was fixing.

Now stated side by side:

Condition	What now - last_commit_time does	Failure
Replica SUSPENDED	last_commit_time freezes, so the delta stops growing exactly when replication stops	Silent - understates at the moment it is worst
Replica healthy but database QUIET	Nothing commits, so the delta grows without bound (measured 1757s at zero real lag)	Loud - pages about an idle database

So guarding only the suspended case does not make a commit-delta trigger safe - it converts a silent failure into a noisy one. And the loud half is the likelier to actually ship, because it shows up the first time anyone points the thing at a database nobody is writing to, whereas the silent half needs someone to suspend something.

The conclusion the file now draws matches the shipped guidance: secondary_lag_seconds is the lag measure (with the finding-3 caveats), and the *_time columns are for showing an operator when something last happened, not for judging whether it is late.

Why this is worth a PR on its own

Both facts were already in the file and both were measured. What was missing was the inference between them - and that inference is the entire basis of a guidance line that has already shipped. An evidence file cited by shipped code should support the conclusion, not just the premises.

Test plan

• Both measurements were taken on the fixture in earlier runs (the freeze under load and idle; 1757s on a SYNCHRONIZED, is_suspended = 0, zero-lag secondary). No new claims - this adds only the synthesis.
• CHANGELOG entry plus link-ref, no duplicate refs, no conflict markers.
• Branched off current dev (97ed4f20, post-Correct the commit-delta guidance: it fails loud, not silent (#991) #1708) and cherry-free; diff is exactly CHANGELOG.md + tools/ag-fixture/VALIDATION.md, so it cannot conflict with anything in flight.

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