Group Scala Steward update PRs; remove em dashes from prose

[russwyte]

Two commits, reviewable independently.

1. Group Scala Steward update PRs

Scala Steward opens one PR per dependency by default. This generates a pullRequests.grouping config so updates land in a handful of PRs instead: one per dependency family, then everything else split by minor-and-patch versus major, then a catch-all.

New zipxStewardGrouping setting, defaulting to ScalaStewardConfig.Defaults, so consumer repos inherit the grouping. Set it to Nil for one PR per dependency. StewardGroup / StewardFilter / ScalaStewardConfig are re-exported from autoImport.

Two non-obvious details drove the shape, both found by reading Steward and action source:

• The action's repo-config input is Steward's global config channel, and it merges ahead of the repo's own .scala-steward.conf. Since the default list ends in a { group = "*" } catch-all, a pullRequests.grouping block in the repo file would be unreachable. Hence a build setting rather than docs pointing at the repo file.
• The action reads repo-config off the runner filesystem, checks nothing out itself, and silently ignores a missing file at the default path. So the generated workflow gains a checkout step, and zipxWorkflowCheck drift-checks .github/.scala-steward.conf. The check failing is what reports a missing config, because Steward never would.

The trailing catch-all is load-bearing: Steward's version filters need both sides of a bump to parse as strict semver, and anything that fails to parse escapes every version-filtered group. Patch bumps ride along in the minor group so a separate patch group cannot re-fragment the PRs this consolidates.

Validated against Steward 0.39.1 directly: seven representative updates produced five PRs, nothing ungrouped.

Also considered and rejected: switching to Dependabot's new sbt ecosystem. It cannot update this repo. Shared version vals in project/Dependencies.scala are never bumped (latest_resolvable_version returns nil for multi-dependency properties and full-unlock is hardcoded false), and its sbt-plugin coordinates are wrong for sbt 2.x (sbt-zipx_3_2.0 404s; the real artifact is sbt-zipx_sbt2_3). Dependabot keeps its existing github-actions job unchanged.

2. Remove em dashes from prose

Repo-wide sweep. Docs, scaladoc, comments, README, and ROADMAP now use colons, commas, semicolons, parentheses, or two sentences. No behavior change except two user-visible strings that contained one: the CI verify-gate echo and the plugin's undiffable-base-ref warning. En dashes in numeric ranges (cron 0-59, 0-23) are left alone as typographically correct.

Verification

• scalafmtCheckAll, scalafmtSbtCheck: pass
• workflow/testFull; core/testFull; central/testFull; docs/testFull: 304 tests, 0 failed, 0 ignored
• plugin/scripted: + zipx/generate-check
• zipxWorkflowCheck: all four generated files up to date

Still to do after merge, since both need the workflow on the default branch: workflow_dispatch the Steward workflow and confirm the log shows the config being picked up, and confirm consumer inheritance from another early-effect repo.