Automate the kindlings macro-timeout canary — a bare-integer spelling silently disables it

[kryptt]

What happened

-Xmacro-settings:{circe,cats,avro}Derivation.timeout=30 had been in build.sbt since the
kindlings 0.3.0 pin and did nothing. kindlings' DerivationTimeout parses the value with

^\s*(\d+)\s*(ms|millis|milliseconds|s|seconds?|m|minutes?)\s*$

and the call site is flatMap(parse).getOrElse(Default) with Default = 5 SECONDS. A bare
integer has no unit, never matches, and is discarded — no warning, no error. Every derivation
had been running on 5s rather than 30s, so the recurring "derivation timed out" CI flake the
setting was added for was never actually mitigated.

Fixed in #102 by spelling every value with its unit (=30s build-wide, =120s for mdoc
fences), verified by canary:

-Xmacro-settings:avroDerivation.timeout=1ms   EXIT=1  Macro … timed out after 1ms
-Xmacro-settings:avroDerivation.timeout=1     EXIT=0  (bare integer: discarded)
-Xmacro-settings:avroDerivation.timeout=30s   EXIT=0

Why this needs a guard

Nothing in the build fails when the key goes dead. The failure mode is silent by construction:
a wrong spelling, a renamed namespace after a kindlings bump, or a dropped setting all look
identical to "everything is fine" until a loaded runner starts flaking again. The build comment
added in #102 documents the canary procedure, but it is a manual step someone has to remember.

Suggested shape

A negative fixture cannot live in the main build — scalacOptions are build-wide, so setting
1ms anywhere would break every module. Options, roughly in order of cost:

• An sbt-scripted test, or a tiny throwaway sub-project outside the root aggregate, that
  compiles one derivation under avroDerivation.timeout=1ms and asserts the compile FAILS with
  timed out after 1ms. Run it in quality.yml rather than on every PR.
• Or a CI step that shells scalac directly against avroIntegration/Test/fullClasspath with
  the 1ms flag and greps the error — no build wiring at all, which is how the build(deps): hearth 0.4.2 / kindlings 0.3.2 — break the 23-element tuple ceiling on .fields #102 canary was
  actually run.

Either way the assertion is the same: the key is live iff 1ms makes a derivation fail.

Related

• build(deps): hearth 0.4.2 / kindlings 0.3.2 — break the 23-element tuple ceiling on .fields #102 (the fix and the measurements)
• The same class of silent-no-op applies to the mdoc fence channel, which already has its own
  documented canary (mdocExtraArguments vs mdoc.properties) — worth covering in the same guard.