docs: correct base-foundryup default-version and release-title claims

[amiecorso]

Summary

Follow-up to the docs cleanup. Two claims in docs/base.md went stale after #46 pinned the bare base-foundryup default to v1.1.0:

• "With no version, base-foundryup installs the latest build" was wrong. A bare base-foundryup now installs the pinned default release (currently v1.1.0, the latest stable Beryl build); rolling nightlies are opt-in via base-foundryup --install nightly. Verified against foundryup/foundryup (FOUNDRYUP_VERSION=${FOUNDRYUP_VERSION:-v1.1.0}).
• "Each release ... is titled with the exact base/base commit it reproduces" was overstated. Only nightlies are titled with the base/base commit (e.g. Nightly (2026-06-23, base a3c3011b)); versioned releases are titled by Base version (e.g. v1.1.0). Verified against the live releases page.
• Minor: "Re-run base-foundryup any time to update" -> "to reinstall or change versions" (the default is pinned, not rolling).

The default-version line is phrased with "currently" so a future default bump doesn't restale it. No other claims in docs/base.md or the README needed changes (audited install flow, the base-* / --base / base = true surface, local-node seeding + chain id, network table, and troubleshooting against the current source).