A floor break can merge with every check green: `lowest-direct` never runs on pull requests

[lesnik512]

#244 shipped a crash to PyPI in 1.8.0 and sat there through 1.9.0. Twelve checks passed on the pull
request that introduced it.

What happened

sentry_instrument.py passed sentry_logs_level=None unconditionally. That parameter arrived in
sentry-sdk 2.25.0; the declared floor is sentry-sdk>=2.1. Every Sentry-enabled service between
those versions failed at bootstrap with TypeError.

scripts/floor_smoke.py catches it. I confirmed that directly: with the pre-fix
sentry_instrument.py installed, floor_smoke.py free fails with exactly that TypeError, and it
passes with the fix.

So the guard existed and worked. It just never ran on the change that needed it.

Why

_checks.yml gates the job on inputs.lowest-direct, which ci.yml leaves at its false default
and only scheduled.yml turns on. The comment explains the reasoning:

Off for PRs: the job resolves direct dependencies at their floor and transitives at their newest,
so a release published upstream today can turn it red for reasons no PR here caused. scheduled.yml
turns it on, where that failure opens a tracking issue instead of blocking a merge.

That reasoning is sound and should not simply be reversed. It protects pull requests from unrelated
upstream drift, which is a real and recurring problem here (#232 was exactly that, from the newest
side).

But it also means a pull request can break a declared floor and merge with every check green,
which is what happened. The daily cadence added in #233 bounds the damage to about a day; it does not
prevent the release. 1.8.0 was tagged the same afternoon.

The tension

The floors job conflates two failure modes that deserve different handling:

1. The diff broke a floor. The author's fault, deterministic, should block the merge.
2. An upstream release broke a floor. Nobody's fault, non-deterministic, should open a tracking
   issue, which is what it does today.

Both look identical to CI right now, so the job is treated as category 2 for everything.

Options

1. Run the floors job on pull requests too, and accept occasional unrelated red. Simplest. Costs
   the protection the current comment was written to provide, and a 25-job matrix on every PR.
2. Run a narrowed floors job on pull requests — one interpreter, only the targets whose files the
   diff touches. Cheaper, and most floor breaks are introduced in the instrument or bootstrapper they
   belong to. Does not help when a shared instrument breaks a framework target's floor.
3. Pin transitives for the PR run so only direct floors move, making category 2 much rarer. The
   floors job already resolves "direct at floor, transitive at newest", which is the part that drifts.
4. Accept and rely on the daily run, and add a release gate: do not tag unless the most recent
   scheduled run is green. Cheap, and directly targets the outcome that actually hurt, which was
   publishing rather than merging.

Recommendation

4 combined with 2, if the narrowing is expressible. The thing that cost users here was not the merge,
it was the tag twenty minutes later. A release gate is a few lines and closes that hole completely;
narrowing the PR job is a bonus that catches it earlier.

Filed rather than fixed because the current behaviour is deliberate and documented, so changing it is
a decision rather than a repair. Related: #236, the same job's other gap, where the floors are
smoke-tested but never type-checked or unit-tested.