spec-h9b Add css preflight checks and a scripts layout block to specs Lined Up

What is wrong (two of the register's five-item spelling-drift bundle apply to specs): (1) specs' `Makefile` has no `css` preflight checks — no `command -v sassc`, no `command -v minify`, no `[ -f $(ASSETS)/scss/base.scss ]` guard — so a missing `base.scss` surfaces only as sassc's one-line import complaint instead of a message naming the package to install. (2) specs' page layout exposes a `{{block "head"}}` seam but not a `{{block "scripts"}}` seam, so a page in specs that needs to add a `<script>` tag has to edit the shared layout instead of overriding a block — the thing the seam exists to prevent.

Why it matters: both are small but compounding — (1) turns a one-line "install sassc" fix into a debugging session for the next contributor without the shared build tools already installed; (2) means the layout keeps growing ad hoc edits every time a page needs a script, instead of staying stable.

Sibling that does it correctly: (1) `sourcehut-bench`, `sourcehut-compare`, `sourcehut-coverage` and `sourcehut-curator` all have the three preflight checks in their `css` recipe with install-hint error messages. (2) siblings that expose both `{{block "head"}}` and `{{block "scripts"}}` in their base layout template.

Fix: (1) add the three preflight checks to specs' `Makefile` `css` recipe, copying the error-message wording from bench/curator. (2) add a `{{block "scripts"}}{{end}}` seam to specs' base layout template alongside the existing `head` block.

Audit: D46 (cross-repo audit, 2026-08-17)